001 Introduction to Immune System
Learning outcome
- Discuss the basic concept of immunology
- Describe the function of immune system
- Describe the types of immunity
- Describe the cells and organs of immune system
1. Introduction
The integrated body system of organs, tissues, cells and cell products that differentiates self from non-self and neutralizes potentially pathogenic organisms.
Must be able to differentiate between material that is a normal component of the body (self) and material that is not native to the body (non-self)
A highly specialized receptors present for discriminating between self and non-self body components
2. The immune system
The body's immune system defenses normally coexist peacefully with cells that carry distinctive "self" marker molecules
But when immune defenders encounter cells or organisms carrying markers that say "foreign" they quickly launch an attack
Anything that can trigger this immune response is called an antigen
An antigen can be a microbe such as a virus or even a part of a microbe
Tissues or cells from another person (except an identical twin) also carry non-self markers and act as antigens
This explains why tissue transplants may be rejected
In abnormal situations, the immune system can mistake self or non-self and launch an attack against the body's own cells or tissues
The result is called an autoimmune disease
Some forms of arthritis and diabetes are autoimmune diseases
The immune system consists of
Innate immunity → Primary response (natural)/[non specific]
Acquired immunity → Secondary response (adaptive)/[specific]
3. Immunity
Innate immunity
- Macrophages
- Granulocytes
- NK cells
- Complement
- Other chemical: lysozyme, HCL
- Characteristics:
- action is immediate
- response is non specific
- response is not enhance on repeated exposure to pathogen
Adaptive immunity
- Humoral
- T cell
- B cell
- Ab
- Complement
- Cell-mediated
- Antigen Presenting Cell (APC)
- Characteristic:
- actions requires days to develop
- response is specific
- response is enhance on repeated exposure to pathogen
4. Anatomy and cells of the immune system
The secret to its success is an elaborate and dynamic communications network
Millions and millions of cells, organized into sets and subsets, gather like clouds of bees swarming around a hive and pass information back and forth
5. Cells of the immune system
The precursor (parent) cell is known as pluripotent haemopoietic stem cell ~origin in the bone marrow
Single precursor cell is capable of giving rise to all blood cell lineages
Most immune cells are WBC
- Granulocytes (N B E)
- Monocytes and dendritic cells
- Lymphocytes
6. Granulocytes
Constitutes approximately 65% of all white cells
Derive their name from the large number of granules in their cytoplasm
Basophils: Granules with intense blue staining
Eosinophils: Red-staining granules
Neutrophils (polymorphonuclear cell): Fine, unstained granules
Circulate in the blood and migrate into the tissue during inflammatory responses
7. Monocytes and dendritic cells
Monocytes form between 5~10% of circulating WBC and have short half-life
They enter the extravascular pool and become resident in the tissues (Macrophages)
Monocyte or macrophages are larger than neutrophils and lymphocytes, have a single nucleus and abundant granular cytoplasm
Examples of specialized macrophages
- Alveolar macrophages (lung)
- Kupffer cell (liver)
- Mesangial cell (kidney)
- Microglial cell (brain)
- Osteoclast (bone)
Dendritic cells: small population, numerous cytoplasmic processes
Bone marrow derived
Highly specialized function in the activation and priming of lymphocytes
8. Lymphocytes
Make up 25~35% of WBCs
Divided into 2 subtypes, B and T
Present in blood in a ratio of approximately 1:5
Lymphocytes are found in the blood, lymphoid organs or tissues and also at sites of chronic inflammation
9. B lymphocytes
Differentiate within the bone marrow before being released into the circulation
Primary role: recognition of antigens through surface receptors (antibodies)
10. T lymphocytes
Thymus-derived lymphocytes
Their involvement with the thymus takes place in early life and critical to their development
Acquire the ability to distinguish self and non-self (graft rejection)
In the absence of TL, protection against infection is fatally impaired
Make a telling contribution to B lymphocyte function
11. Natural Killer Cells
Small populations
Resemble T lymphocytes but remain distinct
Capable lysing virus-infected cells and tumor cells
Best identified by the presence of specialized surface glycoprotein (like lymphocytes)
Typically have a very granular cytoplasm
12. Phagocytosis of Bacteria by Macrophages
13. Organs of the immune system
The organs of the immune system are positioned throughout the body
They are called lymphoid organs because they are home to lymphocytes, small WBCs that are the key players in the immune system
Primary lymphoid organs
- Sites of development and maturation of the lymphocytes
- Bone marrow
- The precise mechanism by which a pluripotent stem cell in the BM matures into any one of the immune cells remains unclear
- Thymus
- Develops in the 6th week of fetal life
- Immature cells enter the cortex, resulting in their development into immature TL → mature TL
Secondary lymphoid organs
- Not essential for the generation of lymphocytes, but have a key role in the maturation of these cells and development of immunity
- 3 major functions
- They are the residence for a variety of lymphoid cells
- They are traps for antigen
- They are the anatomical site in which immune responses are initiated
- Lymph nodes
- Lymphocytes and other migratory cells (e.g: dendritic cells) enter lymph nodes through the lymphatics (via afferent lymphatic channels) or directly from the blood (via HEV structures)
- Cells leave via the efferent lymphatic channels, passing into the thoracic duct and into the venous system
- The presence of secondary follicles with germinal centres indicates an ongoing immune response
- Lymph vessels
- Lymphocytes can travel throughout the body using blood vessels
- The cells can also travel through a system of lymphatic vessels that closely parallels the body's veins and arteries
- Cells and fluids are exchanged between blood and lymphatic vessels, enabling the lymphatic system to monitor the body for invading microbes
- Spleen
- The spleen is a flattened organ at the upper left of the abdomen
- Like the lymph nodes, the spleen contains specialized compartments where immune cells gather and work, and serves as a meeting ground where immune defenses confront antigens
- If the lymph node is an antigen trap for the tissues, the spleen is an antigen trap for the blood
- Blood traffic of cells and proteins can be slowed down and inspected (particularly with regard to infectious agents and Ag-Ab complexes)
- Macroscopic appearance: white pulp and red pulp
- The white pulp is important in immune response, and the red pulp is important for RBC and WBC removal
14. Other lymphatic tissues
Clumps of lymphoid tissue are found in many parts of the body, especially in the linings of the digestive tract, the airways and lungs territories that serve as gateways to the body
These tissues include the tonsils, adenoids, and appendix
002 Innate (Non Specific) Immunity
Learning outcome
- Define innate and adaptive immunity
- Identify innate immune cells and molecules
- Describe functions of phagocytic cells
- Detection of microbes (Pattern recognition receptors)
- Phagocytosis and microbicidal mechanisms
- Secretion of pro-inflammatory proteins (Cytokines)
- Directed migration (Chemotaxis)
- Describe inflammation/leukocyte adhesion
- Describe other innate cells/factors
1. Immune mechanisms
Innate
- Present at birth
- Limited diversity
- Fixed, repeating, broad responses to a limited number of foreign substances
- Nonspecific defense
- Response does not target one specific immunogen
- No memory
- Primary and secondary responses are identical
- Examples: skin, cough, gastric pH, fever, inflammation (phagocytic cells)
- Physical/mechanical barriers
- Intact skin, epithelial layers, cough, fever
- Nonspecific chemical factors
- Antimicrobial peptides and fatty acids, gastric pH, lysozyme
- Inflammation
- Phagocytes (engulf and digest microbes)
- Pro-inflammatory factors (Cytokines, complement proteins)
- Natural Killer cells (nonspecific cytotoxic cells)
- Interferon (produced by virus-infected cells and induces antiviral state in neighboring uninfected cells)
Acquired
- Appears after contact with antigen
- Vast diversity
- Specific defense
- Memory responses
- Examples: antibody, cytotoxic lymphocytes
2. Intrinsic epithelial barriers to infection
- Mechanical
- Epithelial cells joined by tight junctions
- Longitudinal flow of air or fluid across epithelium
- Movement of mucus by cilia
- Chemical
- Fatty acids (skin)
- Enzymes: lysozyme (saliva, sweat, tears), pepsin (gut)
- Low pH (stomach)
- Antibacterial peptides; defensins (skin, gut), cryptidins (intestine)
- Microbiological
- Normal flora compete for nutrients and attachment to epithelium and can produce antibacterial substances
3. Second line body defense
Components
- Cells
- Phagocytes
- Natural Killer cells
- Soluble factors
- Acute phase proteins
- Complement
- Cytokines
- Lysozyme etc.
4. Inflammation
Local accumulation of fluid, plasma proteins and WBC that is initiated by physical injury, infection, or a local immune response
Local, non specific response to tissue damage
Tissue damage may be due to initiating stimulus
- Endogenous factors
- Tissue necrosis
- Bone fracture
- Exogenous factors
- Mechanical injury (cut)
- Physical injury (burn)
- Chemical injury (exposure to corrosive chemical)
- Biological injury (infection by organism)
- Immunologic injury (hypersensitivity reactions)
Key role is played by phagocytes, which engulf the microbes or damaged cells and destroy them
Function as protective response against injury and infection
5. Symptoms of inflammation
Redness is a result of increased blood flow to the infected area
Pain may be caused by the stimulation of pain receptors by kinins, injury of nerve fibers and irritation by toxic chemicals released by microbes
Heat is due to the large amount of warm blood flowing to the area and the increased metabolic activities occurring in the area
Swelling. An increase in the permeability of the capillaries permits more fluid to move from the blood into the tissue spaces
6. How fever happen
Fever means elevated body temperature
It is triggered by microbial substances or cytokines from activated phagocytes which reset thermostat in hypothalamus that accelerates defense mechanisms and repair
It enhances activity of antiviral and antibacterial enzymes
7. Major physiological events during inflammatory response
- Vasoconstriction
- Constriction of blood vessels leading to decreased blood flow to a part
- Vasodilation
- Increased volume of blood flow to the site
- Decreased velocity of blood flow to the site
- Leukocytes are able to slow down and adhere to vascular endothelium
- Increased expression of vascular endothelial adhesion molecules
- Leukocytes are able to attach to vascular endothelium
- Increased vascular permeability
- Fluid enters tissues
- Influx of phagocytic cells into tissues due to increased margination and extravasation
8. Process of inflammation
9. Biological features of inflammatory reaction
Tissue damage due to initiating stimulus
Increase in the permeability of blood vessels in the area. This results in a buildup of the following compounds in the effected tissues
- Leukocytes (WBC)
- Macrophages
- Neutrophils
- Lymphocytes
- Complement
- Vasoactive substances, such as fibrinogen, histamine, kinins
- Cytokines (interferon, interleukins, etc.)
10. Phagocytosis
Phagocytes recognize pathogen-associated molecular patterns (PAMP) via pattern recognition receptors such as CD14/Toll receptors and produce pro-inflammatory cytokines and chemokines
Microbial substances (LPS, LTA, PPG) may directly activate plasma complement proteins stimulating inflammation
Phagocyte functions
- Recognition of microbes (pattern recognition)
- Synthesis and secretion of cytokines/chemokines
- Phagocytosis (binding and engulfment of particles)
- Intralysosomal digestion and killing of ingested material
- Antigen presentation to lymphocytes
- Chemotaxis (migration toward a chemoattractive signal)
Neutrophil (neutrophilic polymorphonuclear leukocyte) in stained blood smear
Neutrophils (PMN) are professional phagocytes
Mononuclear phagocytes (monocytes and macrophages)
- Non opsonic
- Direct engulfment via innate pattern recognition receptors. Slow, limited, inefficient
- Opsonic
- Engulfment of complement-coated or antibody-coated microbes via complement receptors (CR) or antibody receptors (FcR). Rapid, very efficient
11. Phagocyte killing mechanisms
- Oxygen independent
- Powerful microbial agents, eg. Lysosomal hydrolases, lysozyme, lactoferrin, defensins, acid pH
- Oxygen dependent (reactive oxygen and reactive nitrogen intermediates)
- Hydrogen peroxide, superoxide anion hydroxyl radical, hydrochloride
- Reactive Nitrogen Intermediates (RNI)
- Nitric oxide, peroxynitrite
12. Activation of complement
Complement is a group of approximately 25 plasma proteins
Complement can be activated by
- IgG or IgM with immunogen complexes
- bacterial cell wall, toxins
- Pharmacological effect -release of lysosomal enzymes
- Proteolytic activity, Chemotactic, Opsonization, Cellulolysis
13. Vasoactive substances
Vasoactive substances important in initiation and early stages of acute inflammation
Widely distributed in tissues in mast cells
Abundance in organs and structure rich in connective tissues such as mammary glands, tongue, prostate, GIT, RIT, serous membrane, and in the skin
Released by several mechanism:
- Mechanical disruption and cell lysis
- Lysosomal substances from neutrophils
- Activated complement component (C3a)
- IgE -allergen complexes
Histamine stimulate vasodilation and increased capillary permeability
Kinins causes vasodilation and increased capillary permeability
Prostaglandins increases vasodilation, increased capillary permeability, serves as chemoattractant for neutrophils
Leukotrienes increases smooth muscle contraction, serves as chemoattractant for neutrophils
14. Proinflammatory cytokines
Cellular sources: macrophages, infected cells, injured cells
Factors:
- Cytokines: IL-1, 6, 12, TNF-alpha
- Induce fever, acute phase protein synthesis, leukocyte adhesion
- Chemokines: IL-8
- Attracts and recruits neutrophils
15. General properties of cytokines
Cytokine secretion is a self-limited event (transient)
They are potent in minute amounts
One cytokine can act on different cells (pleiotropic)
Multiple cytokines may have the same functional effects (redundant)
Cytokines often influence synthesis and action of other cytokines
Two cytokines may antagonize each other's action (produce additive or synergetic effects)
Action of cytokine may be local or systemic
Cytokine act close to the site of production (autocrine action) or on a nearby cell (paracrine action)
Large amount secretion may enter circulation and act at a distance from site of production (endocrine action)
16. Functions of cytokines
17. Chemotaxis
Dependence on:
- ADHESION MOLECULES (induced by cytokines)
- β2-integrins on leukocyte
- E-selectins on vascular endothelium
- RECEPTORS for Chemoattractants
- CHEMOATTRACTANTS
- Leukotriene B4, Complement C5a, bacterial formyl-methionyl peptides, chemokines (interleukin-8, etc.)
18. Factors which activate and recruit inflammatory phagocytes
Cytokines (released locally) stimulate phagocyte adhesion to vascular endothelium and extravasation (diapedesis)
Chemoattractant compounds direct phagocytes to the site of infection.
“Acute phase” and complement proteins enhance phagocytosis and killing of microbes by recruited “inflammatory” phagocytes
Leukocyte adhesion and diapedesis. Cytokine-induced adhesion molecules and locally-released chemokines promote binding of circulating phagocytes to the vascular endothelium and cause a directed migration to sites of tissue injury or infection.
NK cells exhibit cytotoxicity toward cells that downregulate MHC molecules that are present on normal cells.
Bacteria and other pathogens enter wound. Platelets from blood release blood-clotting proteins at wound site. Mast cells secrete factors that mediate vasodilation and vasoconstriction. Delivery of blood, plasma, and cells to injured area increases. Neutrophils secrete factors that kill and degrade pathogens. Neutrophils and macrophages remove pathogens by phagocytosis. Macrophages secrete hormones called cytokines that attract immune system cells to the site and activate cells involved in tissue repair. Inflammatory response continues until the foreign material is eliminated and the wound is repaired.
19. Therapeutic uses of cytokines
Interferon in treatment of viral diseases and cancer
Several cytokines are used to enhance T-cell activation in immunodeficiency diseases, e.g. IL-2, IFN-ɣ, TNF-α
IL-2 and lymphokine activating killer cells in treatment of cancer
Restore leukocytic count after cytotoxic chemotherapy induced neutropenia
Used to correct AIDS-associated leukopenia
Anti-cytokines antibodies used in management of autoimmune diseases and transplant rejection.
Anti-TNF in used in the treatment of rheumatoid arthritis
Anti-IL2 to reduce graft rejection and adult T-cell leukemia
Anti-TNF antibodies in treating septic shock
Anti-IL-4 is under trial for treatment of allergies
20. Benefits of inflammatory response
The benefit of the inflammatory reaction is that there is a very quick response to the disease agent which may stop further progression of the disease.
The downside of the inflammatory reaction is that it is nonspecific and the inflammatory compounds released are also harmful to the host.
Because of this, inflammation is under tight regulatory control in the body.
003 Adaptive Immunity
Learning outcome
- Define humoral immunity (HI)
- Define cell mediated immunity (CMI)
- List main characteristics of specific immunity
1. Adaptive/Acquired immunity provides specific defenses against infection
Slower responses to specific microbes
Humoral response (Ab)
Cell-mediated response (cytotoxic lymphocytes)
Innate immunity
Rapid responses to a broad range of microbes
External defenses (skin, mucous membrane, secretions)
Internal defenses (phagocytic cells, antimicrobial proteins, inflammatory response, NK cells, fever)
2. Levels of defense against infection
3. Third line of defense (specific or adaptive immunity)
Humoral
involving specific lymphocytes (B lymphocyte and antibodies)
Synthesis of specific molecules by B cells
Antibodies which are subsequently found in body fluids such as the blood and lymph
Cell mediated
involving specific lymphocytes (T lymphocytes and cytotoxic cells)
involves activation of T-cells, neutrophils, macrophages and cytotoxic T-cells.
Main characteristics
- Specificity
- Diversity
- Memory
- Self/non self
Immunity is triggered to specific antigens
Not restricted to infection site
Has memory therefore faster response to antigen
Help determine what is foreign
Stimulates T and B cells and causes production of antibodies
Antibodies: specific protein produced by B cells in response to an antigen (Ag); specific Ab bind to specific Ag and form Ag-Ab complexes (foreign cell is immobilized and is easier for phagocytes to destroy), most are found in the gamma globulin component of plasma proteins
Lymphocyte development
produced in red bone marrow
T and B cells are immunocompetent when they display unique receptor and can bind to an antigen
B cells become immunocompetent in the bone marrow
T cells migrate to the thymus and become immunocompetent
Both disperse to the lymph nodes and the spleen
4. Specific Immunity
Lymphocytes provide specificity and diversity of immune system
Antigens interact with specific lymphocytes inducing immune response and immunological memory
- Ability to distinguish self from non-self
- Immune tolerance for self
- Memory for previously encountered Ags
- Provokes more rapid and vigorous response
- Cell surface markers in T cells function and development
- Major histocompatibility complex
- Self recognition
- MHC I
- possessed by all nucleated cells
- MHC II
- macrophages, B cells and some T cells
5. Maturation of T and B cells
Immunocompetence is the ability to carry out immune response
Cells develop from pluripotent stem cells of red bone marrow
B cells develop in red bone marrow through life
T cells mature in thymus
- Antigen receptors
- Able to recognize specific antigen
- CD4/CD8
6. Types of immunity
Innate immunity
Phagocytes
Early rapid responses, but limited and non-specific
Adaptive immunity
Lymphocytes
Take time but powerful "specificity + memory"
Measles attacks and immunological memory
In the first attack, adaptive immunity is too slow to prevent the virus growing and causing symptoms
In the second attack, an Ab response in made so rapidly that the virus is disposed of before symptoms appear
7. Memory in adaptive immunity
1st infection
Slow response
Pathogen proliferate
Disease
Symptoms
Memory
2nd infection
Fast response
Pathogen killed
No disease
No symptoms
8. Antigens and antigen receptors
Immunogenicity
- Ability to provoke immune response by
- Stimulation production of specific antibodies
- Proliferation of specific T cells
Antigen-antibody generator
Reactivity
- ability of antigen react specifically with antibodies or cells it has provoked
Epitope/antigenic determinants
- small part of a large Ag which acts as trigger for immune response
Antigen route after successfully bypassing nonspecific defenses
- Through blood stream
- Trapped in the spleen
- Penetrate the skin
- Enter lymphatic vessels and lodge in nodes
- Through the mucous membrane
- Trapped by MALT (mucosa-associated lymphoid tissue)
9. Chemical Nature of Antigens
Large complex macromolecules
Not large repetitive subunit molecules (polysacchrides)
Hapten
Smaller substance with reactivity but lacks immunogenicity
Able to stimulate response only if attached to larger carrier molecule
10. Major Histocompatibility Complex (MHC) Antigens
In plasma membrane of body cells
Unique except identical twins
Helper T cells recognize foreign Ag
Antigen processing
B cells able to bind in fluids without presentation or requirements
T cells
Fragments with MHC complex (antigen-MHC complex)
Ag presentation
Antigen presentation
11. Exogenous antigens
Antigens from outside the body
APCs include macrophages, B cells and dendritic cells
Processing and presenting exogenous antigens
- Phagocytosis of antigens
- Digestion of antigen into peptide fragments
- Synthesis of MHC molecules
- Fusion of vesicles
- Binding of peptide fragments to MHC molecules
- Insertion of Ag-MHC complex into plasma membrane
12. Endogenous antigens
Synthesized in the body cells
Ag-MHC complex
Movement to plasma membrane
Cell signals for immune system to respond
004 Specific Immune Recognition
The Antibody Molecule
1. Defense system
2. Adaptive Immune System
Specificity – Recognition of particular antigens
Memory – Remembers previously encountered antigens
Systemic – Immunity is not restricted to the initial infection site
Immune responses
– Antibody-mediated or humoral immune responses (late 1800s)
– Cell-mediated immune responses (mid 1900s)
3. Acquired/Adaptive Immunity
Naturally acquired
- Active
- Infection; contact with pathogen
- Passive
- Ab pass from mother to fetus via placenta; or to infant by breastfeeding
Artificially acquired
- Active
- Vaccine; dead or attenuated pathogens
- Passive
- Injection of immune serum (gamma globulin)
4. Introduction to the specific immune system (IS)
Outcome of innate IS: able to eliminate pathogen or not.
If cannot, the answer is antibody (Ab) or immunogloblin (Ig). Both terms (Ab & Ig) mean the same thing.
Antibody is vital for human life. No Ab, you die.
5. Antibody structure
Y-shaped structure consisting of 4 protein subunits.
Two longer subunits called heavy (H) chains. Two shorter subunits called light (L) chains.
The H chains are linked to each other, and to the L chains, by disulphate bridges.
6. Antigens and antigen receptors
Antigens (Ag) can be entire microbes, parts of microbes or chemical components of pollen, egg white, blood cells, etc.
Complete antigens
– Immunogenicity
– Reactivity
Incomplete antigens
– Haptens
– Epitopes
Lock-and-key
7. The Ag binding sites of Abs
Called Ag-binding sites and are located in the variable domains of the H and L chains (also called Fab).
Abs against different Ags have different amino acid sequences.
Variable and hyper-variable regions.
– Variable: do not differ so much between Abs (framework regions)
– Hyper-variable: determine whether it is complementary to an antigenic epitope. Also called complementarity-determining regions (CDRs).
8. The nature of Ag-Ab binding specific recognition
Non covalent:
– Hydrogen bonds, electrostatic forces, van der Waals forces and hydrophobic forces.
– Weak; thus a lot of these interactions are required for strong binding.
– There must be a good fit between the Ab-binding site & the Ag epitope.
– The binding strength is called affinity.
High affinity = strong binding.Ab can distinguish Ag from host cell even only one amino acid difference!
9. Affinity and avidity of Ag-Ab interactions
Affinity: the strength with which an individual binding site of an Ab binds to its epitope.
Avidity: the overall strength of binding.
To break the interaction between multiple binding sites it is necessary to break the binding at every binding site at the same time → requires more energy.
10. Classes of antibody
Immunoglobulin-G (IgG)
• The smallest antibody.
• Found in all body fluids.
• Produced by B cells in response to foreign antigen.
• 80-85% of the total serum immunoglobulin.
• Involved in the secondary immune response.
• The only isotype that can pass through the placenta.
• Also present in colostrum.
• Responsible for protection of the newborn.
• Circulates in the blood and exits the vessels into tissues.
• Capable of binding complement which will result in cell lysis.
Immunoglobulin-M (IgM)
• The largest antibody (Pentamer).
• Found in blood and lymph fluid.
• 5-13% of the circulating antibodies.
• Involved in primary immune response.
• Responsible for neutralization antigen.
• Control bloodstream infections.
• Very effective in agglutination and precipitation.
Immunoglobulin-A (IgA)
• Produced in mucosal lining.
• Found in mucous secretions.
• 10-13% in the serum.
• Prevents colonization by pathogens.
• Have two subclasses, IgA1 and IgA2.
• The secretory component of IgA protect the immunoglobulin from being degraded by proteolytic enzymes.
• It is a poor activator of the complement system and opsonises only weakly.
• Present in LARGE quantities in breast milk which transfers across gut of infant.
Immunoglobulin-E (IgE)
• Found only in mammals.
• Barely detectable in normal blood.
• Defends against parasitic invasion.
• Responsible for allergic reactions.
– Fc region binds strongly to mast cells
– Mediates release of histamines and heparin>allergic reaction
Immunoglobulin-D (IgD)
• Makes up about 1% of protein in the plasma membranes of mature B-lymphocytes.
• Produced in a secreted form in small amounts in blood serum.
• Involved with the development and maturation of the antibody response (Surface Ab).
• Functions in serum have not been clearly described.
11. Ab can secreted or expressed on the cell surface of B lymphocytes
IgM and IgD
So, Ab can exist in two forms, a soluble form that has a number of biological activities (e.g. IgG, IgM. IgA, IgE, IgD);
And an integral cell-membrane protein on the surface of B-lymphocytes – for B lymphocytes to recognize specific Ag and respond to it.
12. Dynamics of antibody production
Antibody production
– Initial antibody produced in IgM
– Lasts 10-12 days
– Followed by production of IgG
– Lasts 4-5 days
– Without continued antigenic challenge antibody levels drop off, although IgG may continue to be produced.
13. Secondary response
Second exposure to SAME antigen.
Memory cells are a beautiful thing.
Recognition of antigen is immediate.
Results in immediate production of protective antibody, mainly IgG but may see some IgM
Antibody response to infection
004 Anatomical and cellular aspects of antibody production
Learning outcome
- To learn how antibody is produced in vivo in response to antigen.
- To know about the role of CD4 T cells & cytokines in antibody production.
- To understand how B cells switch the class of antibody they make and increase the affinity.
- To know that B cells can become either plasma cells or memory B cells.
1. Overview of Ab production
- Ab is produced by plasma cells that differentiate from B cells.
- B lymphocytes that have never encountered antigen before – have IgM and IgD on their surface (they can recognize and bind Ag through the IgM & IgD but still cannot secrete Ab).
- Different B cells will have different specificity for different Ag.
- To become plasma cells, B cells must undergo a process of cell differentiation.
- An important cell that regulates or helps B cells differentiation is CD4 T cell or helper T cell (Th for short).
- Another important feature for Ab production is proliferation of B cells (clonal proliferation)
- Therefore..
- The production of antibody can be divided into 3 stages:
- Presentation of Ag to, and stimulation of, antigen-specific CD4 T cells to proliferate & differentiate into Th.
- Stimulation of B cells by Ag and interaction with Th.
- Proliferation of B cells & their differentiation into plasma or memory cells.
2. ACTIVATION OF CD4 T CELLS (0 – 5 DAYS)
- The first, and critical stage.
- Stimulation of CD4 T cell to become Th.
- Occurs at lymph nodes & spleen.
- The production of Th can be considered in three stages:
- Delivery of antigen to lymph nodes or spleen
- When the tissue is infected by microbe, dendritic cells take up the antigen (i.e. by phagocytosis), leave infectious site & migrate to the lymph nodes.
- As they migrate, they process the Ag, results in the expression of class II MHC on their surfaces for recognition by CD4 T cells at lymph nodes.
- Similar way in spleen, except spleen will deal with blood pathogens.
- Activation of CD4 T cells
- The CD4 T cell should be able to recognize & respond to as few molecules of Ag/class II MHC as possible ~ so that the response can be initiated soon after infection.
- Involves the interaction of FcR on CD4 T cell surface with class II MHC on Ag-presenting cell (dendritic cell), and the reaction of co-stimulus as well.
- Proliferation and differentiation of CD4 T cells
- After activation by antigen-presenting dendritic cells.
- The proliferation of CD4 T cells is driven by cytokines (IL-2).
- This proliferation peaks 3-4 days after initial contact with Ag, results in ~ 10,000- to 100,000-fold increase.
- Following the period of proliferation, CD4 T cells begin to differentiate into Th cells.
- Next, these Th cells will move to cortex area of lymph nodes and interact with B cells.
3. STIMULATION OF B CELLS BY ANTIGEN & INTERACTION WITH Th
- Antigen filtering through the lymph node cortex captured by macrophages.
- If the Ag on macrophage comes into contact with B cells whose Ig is specific for, this event can occur:
- B cell take up the Ag, process class II MHC/Ag complex.
- These activated B cells will interact with Th cell through their TcR.
- The Th will be stimulated and produce cytokines.
- Some of B cells differentiate into plasma cells secreting IgM.
- Some more undergo class switch to IgG (and this will differentiate into IgG-secreting plasma cells).
- After 4-7 days, some of the B cells and Th cells migrate to primary follicles to form germinal centre.
- almost similar way in spleen.
4. FORMATION OF GERMINAL CENTRES (4-14 DAYS)
Germinal centre is the specialized structure formed within lymphoid tissue following an encounter with Ag.
Four main events occur at germinal centres:
– Antibody class switch
B cell is able to change the heavy chain constant region, thus change the Ig class.
Class switch is controlled by cytokines and Th.
Cytokines also affect the level of antibody production by plasma cells.
– Affinity maturation of Ab
Important to produce antibody with high affinity for Ag because high-affinity Ab works better.
Due to hyper-mutation of heavy and light chain variable genes of the Ab change the amino acid sequence change the conformation of the Ag binding site.
At the end, only Ab with high affinity will survive, lower affinity Ab will be rejected.
Differentiation into plasma or memory B cells and Ab production.
– Differentiation of B cells into memory cells
Final stage of B cell differentiation.
Plasma cells secrete large amount of Ab while memory B cells provide protection against future infection.
Memory B cells are long-lived and can survive long after an initial infection has been eliminated.
Memory B cells enter the recirculating lymphocyte pool & migrate thru’ lymph nodes, spleen and some other lymphoid tissue.
If they encounter Ag in the future they are able to very rapidly develop into plasma cells and secreting high affinity IgG, IgA or IgE because they do not need to undergo affinity maturation or class switch again.
– Differentiation of B cells into plasma cells
Plasma cells can be short- or long-lived.
Some stay in lymphoid tissues where they are produced (tend to be relatively short-lived and secrete Ab for few weeks).
Other plasma cells migrate to the bone marrow, interact with stromal cells, and there they secrete Ab for many months or years.
005 T lymphocytes and MHC-associated recognition of antigen
Learning outcome
- To know the structure of the MHC and the roles of MHC molecules in antigen presentation to T cells.
- To know how T cells recognize antigen.
- To learn about antigen processing.
- To appreciate the unique polymorphism of class I and class II MHC molecules and the advantages of this.
1. Overview of T lymphocyte subsets
You have learned about the antibody, which can be secreted in soluble form or be present on the surface of B lymphocytes.
Through Ab on their cell surface, B lymphocytes can recognize Ag and trigger a cellular response.
Another type of lymphocyte which is T lymphocytes can also recognize Ag, but in a different way.
There are two types of TL:
– CD4 T cells (or helper T cells).
– CD8 T cells (or cytotoxic T cells).
2. The T cell receptor for antigen
Both CD4 and CD8 T cells use a receptor called T cell receptor (TcR) to recognize Ag.
The TcR is never secreted like Ab but exists only as a receptor on the surface of T cells.
Different T cells will recognize different Ag and each T cell will express only one specificity.
T cells do not recognize free Ag in the way Ab can.
They recognize Ag that is associated with molecules on the surface of cells called major histocompatibility complex (MHC) molecules.
3. Class I and class II MHC
Class I: Expressed on the cell surfaces of most nucleated cells.
Class II: More restricted in their expression.
– Focus primarily on cells of the immune systems such as monocytes / macrophages and B cells.
Both classes show high degree of polymorphism.
Both perform a similar function.
4. Recognition of Ag by T cells
The complexes of peptides-MHC are sticking out from the cell surface so that they are easily accessible to the TcR on a T cell.
The TcR can bind to these complex of peptide-MHC.
CD4 T cells recognize Ag presented by class II MHC molecules and CD8 T cells recognize Ag presented by class I MHC molecules.
The binding of TcR to peptide-MHC complex involves non-covalent bonds.
5. Antigen processing and presentation by MHC molecules
How does the antigenic peptide get to the MHC molecule?
The answer:
– Large protein antigens are broken down into peptides inside cells.
– These peptides associate with MHC molecules intracellularly.
– The MHC molecules bearing the antigenic peptides are then transported and expressed on the cell surface.
Endogenous antigens are produced within the cell (e.g. viral proteins) and are processed & presented by class I MHC.
Exogenous antigens derive from outside the cell (e.g. from a bacterium or parasite) and are processed & presented by class II MHC.
6. Presentation of endogenous antigen by class I MHC
When a virus infects a cell its DNA directs the production of viral proteins within the cytoplasm of the cell ~ so they
are inaccessible to Ab.
However, peptides derived from these viral proteins can be presented by class I MHC.
How??
Proteosomes degrade cytoplasmic antigens
– Proteosomes have proteolytic activity & are able to degrade proteins into peptide fragments.
Transporter carry antigenic peptides into the rough endoplasmic reticulum
– TAP transporter transport the peptide fragments from cytoplasm to RER where MHC molecules are synthesized.
Antigenic peptides are required for the correct assembly of class I MHC.
– Stable MHC molecule requires the presence of an antigenic peptide to be expressed and presented to CD8 T cells.
7. Presentation of exogenous antigen by class II MHC
Class II MHC molecules present antigenic peptides that are derived from antigens outside the cell.
The pathway for this is quite different from that for class I MHC.
Extracellular proteins are endocytosed and degraded in lysosomes
– Endocytosis could be by pinocytosis, receptor-mediated endocytosis or phagocytosis.
– Fuses with lysosomes to form endolysosome → degraded into smaller fragments.
Antigenic peptides associate with class II MHC in specialised compartments.
– The class II MHC molecule bearing the antigenic peptide is then transported to the cell surface, expressed and presented to CD4 T cells.
8. Important aspects of antigen processing and presentation
One way of rationalizing the development of two different pathways
is that each ultimately stimulates the population of T cells that is most effective in eliminating that type of antigen.
Viruses replicate within nucleated cells in the cytosol and produce endogenous antigens that can associate with class I MHC. By killing these infected cells, cytolytic T cells help to control the spread of the virus.
Bacteria mainly reside and replicate extracellularly. By being taken up and fragmented inside cells as exogenous antigens that can associate with class II MHC molecules, helper T cells can be activated to assist B cells to make antibody against bacteria, which limits the growth of these organisms.
006 Antibody-mediated responses
Learning outcome
- To know about the different ways the specific immune responses aids in neutralising or eliminating pathogens.
- To understand the different ways in which Ab provides protection from infectious agents.
- To learn about the components and biology of the complement system.
1. Antibody-mediated effector responses
Mostly involve interaction with components of the innate IS.
– Neutralisation
– Agglutination
– Opsonisation
– Activation of complement
– Ab-dependent cell-mediated cytotoxicity
2. Neutralization by antibody
• Toxins
– Binding to them & inhibiting their action.
– E.g. tetanus, diphteria and botulism.
• Virus
– Ab can bind to the viral receptor molecule & stop it binding to the cell, thereby preventing infection.
– Can also prevent the virus from replicating after it has entered the cell, e.g. measles & influenza virus.
• Bacteria
– Ab can affect bacterial adherence to cells & inhibit their metabolism.
– E.g. mucosal IgA can bind cholera bacteria & prevent their adherence to intestinal epit.
3. Agglutination
• Abs are multivalent, so they are able to bind to more than 1 microbial particle and form complexes (known as agglutination).
• Agglutination can limit the spread of
pathogens by retaining them in clumps.
• Larger complexes of Ag are more likely to be phagocytosed and killed.
4. Phagocytosis and killing
Recognition & attachment
– Phagocytes can recognise microbes directly.
– However, many pathogens have evolved ways of avoiding direct recognition by phagocytes.
– Opsonins are molecules that bind to pathogens & phagocytes & promote phagocytosis.
– Abs can act directly as opsonins.
• Fab Ag binding sites VS Fc.
Ingestion
– Once triggered, the phagocyte extends its membrane around the particle (pseudopodia).
– Eventually the particle becomes completely surrounded in a phagocytic vacuole.
Killing
– Inside a phagocytic vacuole.
– Phagocytes have a number of mechanisms for killing.
– Some do not require fusion of the phagosome with a lysosome (lysosome-independent).
– But usually are lysosome-dependent killing mechanisms.
– Lysosome-independent killing mechanisms:
• Two main pathways
– Generation of oxygen radicals.
– Production of nitric oxide.
– Lysosome-dependent killing mechanism:
• Normal circumstances.
• Exposes the contents of the phagosome to lysosomal products (toxic / microbicidal).
– Generation of chlorine products.
– Defensins.
– Proteolytic enzymes. E.g. lysozyme.
Degradation
– Microbial products degraded to be excreted by the phagocyte.
– Degradation of microbial proteins can also generate antigenic peptides that can be presented on the surface of the macrophage → stimulating CD4 T cells.
5. Complement
• Another important function of some antibody is to activate complement system.
• A cascade of proteins (like clotting pathway).
• Consists of a series of inactive precursor → activated→ activate next protein in sequence.
• Three complement pathways exist:
– Classical
– Lectin
– Alternative
• The classical complement pathway
– The main components are proteins called C1 to C9.
• Binding of C1 to Ab-Ag complexes initiates the classical pathway.
– C1 binds to the Fc portion of the Ab (known as complement fixation).
– C1 subunits consist of C1q, C1r and C1s.
– The binding subunit is C1q. Once binding occur, C1r is activated (cleaves itself) to form C1r** (active).
– C1r** cleave C1s, leading to activation of C1s**.
• Activated C1 generates products from C4 and C2
– C1s** then cleaves another complement component, C4, into C4a and C4b.
– The C4b attach to the cell membrane and the C4a diffuses away.
– The C4b can now bind the C2, and when C2 is bound, it will be cleaved by C1s** into C2a and C2b.
– The C2b diffuses away but the C2a remains bound to C4b.
– The combination (C4b.C2a) will form active enzyme complex C4b.C2a**.
• C4b.C2a is a C3 convertase.
– The substrate of C4b.C2a** is C3.
– C3 binds to C4b.C2a** and is cleaved into C3a and C3b.
– C3a diffuses away. The C3b generated has two important functions:
i. Acts as an opsonin, promoting the phagocytosis of the cell.
ii. Some of the C3b binds to the C4b.C2a**, forming C4b.C2a.C3b**.
• C4b.C2a.C3b is a C5 convertase.
– Cleaves C5 into C5a and C5b. C5a diffuses away and the C5b binds to the cell surface.
– Note: this is the last enzymatic step in the complement pathway. The later steps are involved in generating pores
(membrane attack complex) that will result in lysis of the cell.
– Also, the subsequent stages are the same for all the pathways (classical, lectin & alternative).
• C6 – C9 are also involved in the formation of the membrane attack complex (MAC).
– C5b is quite labile & has about 2 minutes to bind to the next complement component, C6, before it is inactivated.
– The C5b.C6 complex binds C7 and C8 to form C5b.C6.C7.C8 complex.
– The C5b.C6.C7.C8 complex can form small pores in the membrane, which can result in the lysis of some microorganism.
– However, most cells are not lysed by C5b.C6.C7.C8 and require final complement component, C9.
– One C5b.C6.C7.C8 complex can add up to 16 C9 molecules. They (C9) assemble to form larger pore (MAC) → Lysis will occur due to osmotic imbalance.
• Lectin Pathway
– Very similar to the classical pathway (CP) EXCEPT for the very 1st steps.
• Instead of the binding of C1 to Ab-Ag complexes like in classical pathway,
here, mannose binding protein (MBP) (which always present in serum) binds to mannose residues on the surface of pathogens.
• When MBP has bound to the pathogen, a protease called MASP binds to the MBP.
• The MBP-MASP complex cleaves C4 into C4a and C4b, and C2 into C2a and C2b (same way like C1s** in CP).
• Once C4b is generated, it binds to the cell surface and the rest is the same as in the CP.
• Alternative pathway (AP) of complement
– C1, C4 and C2 are not involved in the AP.
– However, C3 and C5 – C9 are involved in all pathways.
– AP uses different components to generate C3 and C5 convertases.
– Formation of C3 convertase
• C3 in serum is quite labile → spontaneous hydrolysis (low level only) → forms C3a and C3b.
• Some of the C3b binds to the body’s own cells → inactivated by surface membrane regulatory proteins.
• Many microbes lack these regulatory proteins & therefore C3b bound to microbial membrane is not inactivated (mechanism by which innate immune system can distinguish self from non self) → the microbes will be opsonised or lysed by the AP of complement.
• If C3b is not inactivated it binds another component of AP, factor B.
• Factor B bound to C3b then cleaved by another AP protein, factor D → becomes Ba and Bb.
• Ba diffuses away but Bb remains bound to C3b, forming active C3 convertase (C3b.Bb**) which is equivalent to C4b.C2a** in CP.
• C3b.Bb** can cleave more C3, generating more C3b that can bind to microbial membrane.
– Formation of C5 convertase
• Some of the C3b generated actually binds to the C3b.Bb to form C3b.Bb.C3b, which is a C5 convertase equivalent to C4b.C2a.C3b in CP.
• C3b.Bb.C3b cleaves C5 into the same C5a and C5b components as C4b.C2a.C3b.
• Once C5a and C5b have been generated, the latter stages proceed in
exactly the same way as for the CP (formation of MAC).
6. Ab-dependent cell-mediated cytotoxicity (ADCC)
• Many cell types have Fc receptor (FcR) on their surface and therefore able to bind the Fc part of Ab.
E.g. neutrophils, macrophage / monocytes, eosinophils and NK cells.
• Note: ADCC is not the same as phagocytosis.
• The target for ADCC are generally too big to be phagocytosed and the killing is extracellular.
7. Compare ADCC and phagocytosis
007 Cell-mediated responses
Learning outcome
- To know how cytotoxic CD8 T cells are
- generated and kill infected host cells.
- To understand the events contributing to a delayed-type hypersensitivity reaction.
- To appreciate the differing cost to the host of the different types of effector response.
1. Introduction
- There are some infectious agents where Ab is not effective like intracellular pathogens.
- Ab can only limit the spread of these
- organisms during extracellular phase and provide protection against future infection.
- Cell-mediated immunity ~ do not involve Ab
- Production of CD8 cytotoxic T cells
- Generation of delayed-type hypersensitivity
2. Intracellular pathogens
• Bacteria
– Mycobacterium spp., Legionella pneumophilia, Listeria monocytogenes, chlamydia trachomatis.
• Fungi
– Pneumocystis carinii
– Cryptococcus neoformans
• Viruses
– Herpes simplex virus
– Measles virus
3. Cytotoxic T lymphocytes (CTL)
• Also known as CD8 T cells
– Able to recognise antigenic peptides presented by class I MHC on cell surface.
• CD8 T cells that have not been stimulated by Ag before (naïve) are not cytotoxic.
– They have to proliferate & differentiate after their first encounter with antigenic peptides/class I MHC to become CTL.
4. Mechanisms of CD8 T cell-mediated cytotoxicity
• CTL can kill by two different mechanisms:
– Granule exocytosis
– CTL contain granules within their cytoplasm.
– These granules contain a number of proteins that can cause lysis of target cells.
• Perforin: Homologous to complement component C9 that are able to form pores in cell membranes.
• Granzymes: Cleave protein serine residues.
– The process can be divided into four stages:
1. Recognition and binding of the target cell.
▪ CTL must recognise antigen / class I MHC through specific binding of its T cell receptor (TcR).
2. Delivery of a lethal hit.
▪ The granules in the T cell move toward the site of attachment to the target cell.
▪ These granules fuse with the membrane of the CTL & the contents of the granules released.
3. Death of the target cell.
▪ Perforin inserts into the target cell membrane and polymerise to form pores.
▪ Granzymes enter the target cell through these pores, cleave proteins ~ results in the activation of apoptosis (or
programmed cell death).
4. Recycling of the cytotoxic T cell
▪ Once the job has done, CTL can detach from the target cell and can kill other target cell bearing the antigen / class I
MHC.
– Fas pathway (Fas-mediated cytotoxicity)
– Fas = death molecule, expressed on many cell types.
– Must interact with Fas-ligand (Fas-L) in Tcsto activate apoptosis.*
– Binding of TcR of CD8 cytotoxic T cells to its specific Ag/MHC class I cause expression of Fas-L.
– Fas on the target cell will be cross linked by Fas-L on CTL, and apoptosis will be triggered.
– Not all cells express Fas ~ so this mechanism is not effective against all cells.
5. Delayed-type hypersensitivity
• A.K.A Type IV hypersensitivity
• Often generated against pathogens that live inside macrophages themselves.
• The aims of DTH are to:
– Recruit monocytes to the site of infection
– Keep monocytes and tissue macrophages at the site of infection
– Activate the monocytes and macrophages to kill the intracellular organisms.
• Cellular features of the DTH reaction
1. Activation of CD4 T cells
▪ Pathogens living in a tissue will shed Ag, which can be picked up by tissue dendritic cells (DCs), leave the tissue site and migrate via lymphatic vessels to the lymph nodes.
▪ While migrating, DCs process the antigen and expressing class II MHC molecule/antigen complex on their surfaces.
▪ When arrive in lymph node, the MHC class II/antigen complex on the surface of DCs activates CD4 T cells specific to them.
▪ CD4 T cells proliferate & differentiate into Th1 (T helper type I) cells.
2. Migration of effector Th cells
▪ Th cells leave the lymph node, entering the bloodstream and enter tissue (DTH site).
▪ TNFα and IL-2 made by macrophages & Th cells which stimulate local endothelial cells to express adhesion molecules (increase the recruitment of monocytes and Th cells from the blood).
3. Recruitment and retention of monocytes
▪ Monocytes that are recruited to the site of a DTH reaction are subsequently stimulated by cytokines to differentiate into macrophages.
4. Activation of macrophages
▪ The Th cells secrete cytokines (IFNγ, TNFα and IL-2) that activate the macrophages.
▪ The activated macrophages up-regulate class II MHC expression on their cell surface.
▪ The Th cells are stimulated by recognising antigen/class II MHC expressed by macrophages & secrete more cytokines ~ further activate the
macrophages (the response amplified).
5. Elimination of pathogens in DTH reactions
▪ The main reason for activating the macrophages is to increase their ability to kill the intracellular pathogens.
▪ Activated macrophages have increased levels of nitric oxide & oxygen radicals, and higher secretion of proteolytic enzymes, which will kill the pathogens.
6. Different effector responses have different costs to the hosts
• Different types of responses can result in more or less damage to the host’s own cells/tissues.
– Ab responses are generally least damaging.
– Cytotoxic T cell responses are directed at infected host cells, therefore involve a degree of damage to the host.
• In some cases, the cost of killing virally infected cells may be too high, e.g. hepatitis.
– DTH responses carry the greatest potential risk of damage ~may lead to the formation of granulomas (cancer) if chronic.
008 Hypersensitivity
Learning outcome
- Define hypersensitivity reactions, including anaphylactic reactions
- Compares Type I, II, III, and IV hypersensitivity reactions
- Describe skin prick test
1. Introduction
While reactions between antigens and the immune system are important protective defense, they also have the potential of injuring normal tissues and producing disease.
Disorders that result from uncontrolled, exaggerated, or misdirected immune responses are called hypersensitivity diseases.
Hypersensitivity can involve either humoral or cell-mediated responses.
2. Definition and types
Hypersensitivity or allergy is an immune response results in exaggerated reactions harmful to the host.
There are four types of hypersensitivity reactions: Type I, Type II, Type III, and Type IV.
-Types I, II and III are antibody-mediated
-Type IV is cell-mediated
3. Hypersensitivity
Hypersensitivity refers to undesirable (damaging, discomfort-producing and sometimes fatal) reactions produced by the normal immune system.
Hypersensitivity reactions require a pre-sensitized (immune) state of the host.
Hypersensitivity reactions can be divided into four types: based on the mechanisms involved and time taken for the reaction.
Frequently, a particular clinical condition (disease) may involve more than one type of reaction.
4. Allergens
- Inhalant
- pollens, house dusts, feathers and hairs of house pets.
- Ingestants
- foods proteins, drugs, drugs additives such as carbohydrates, fats, cross reacting allergens, tend to diminish with age.
- Drugs
- as allergens or as haptens (incomplete immunogens) - antisera, vaccines.
- Injectants
- injected drugs, diagnostic reagents, insect venom.
- Contactants
- drugs, paints, plants resins, preservative and metals.
5. Classification
Type I (Ige mediated)
2~30 min
Ag induces cross linkage of IgE bound to mast cells and basophils with release of vasoactive mediators
Hay fever, asthma, food allergies, eczema at skin
Type II (antibody mediated)
5~8 hours
Ab directed against cell-surface antigens mediates cell destruction via C’ or Antibody-dependent-Cellular-Cytotoxicity (ADCC)
Blood transfusion, erythroblastosis fetalis, autoimmune hemolytic anemia (AIHA)
Type III (immune complex mediated)
2~8 hours
Ag-Ab complexes deposited in various tissues induces complement activation and ensuring inflammatory response
Arthritis reactions, glomerulonephritis, rheumatoid arthritis, systemic lupus erythematous
Type IV (cell mediated)
24~72 hours
Sensitised TH cells release cytokines that activate macrophages TC cells which mediate cellular damage
Contact dermatitis, tubercular lesion, graft rejection
6. Type I hypersensitivity
Also known as immediate or anaphylactic hypersensitivity.
The reaction may cause a range of symptoms from minor inconvenience to death.
Immediate hypersensitivity is mediated by IgE. The primary cellular component in this hypersensitivity is the mast cell or basophil.
Histamine causes blood vessels to dilate and become 'leaky', and is the main cause of typical allergic reactions - runny nose, watery eyes and itchy, red skin. Symptoms also depend upon where the allergen enters the body. An inhaled allergen causes the airways to constrict, causing asthma symptoms; if ingested, symptoms include cramp, vomiting and diarrhea.
A second, more dramatic, reaction can occur if an allergen enters the bloodstream. (anaphylactic shock). The airways constrict (and the tongue may swell) making breathing difficult, and the sudden dilation of blood vessels and loss of fluid may cause circulatory collapse.
An antigen reacts with cell fixed antibody (Ig E) leading to release of soluble molecules an antigen (allergen) soluble molecules (mediators).
Soluble molecules cause the manifestation of disease.
Systemic life threatening leads to anaphylactic shock.
Local atopic allergies leads to bronchial asthma, hay fever and food allergies.
Immunological basis of allergy
Sensitization
- Production of IgE in response to allergen.
- Dependent on CD4 T cells that respond to the allergens & differentiate into Th
- Th secrete cytokine (IL-4) which is important for production of IgE by B cells.
- The IgE Fc bind to mast cell FcR (mast cells are found in most tissues).
- IgE can stay bound for months or years.
- The initial exposure to allergens usually not involves any symptoms.
Mast cell activation
- Upon re-exposure to allergy, the allergen will bind to allergen-specific IgE that is bound to the FcR on mast cells ~ lead to mast cell activation.
- Consequence: degranulation & the release of preformed mediators from the mast cell granules (e.g. histamines & TNFα).
- Followed by synthesis & release of newly formed mediators (e.g. prostaglandins & leukotrienes).
- These mediators cause vasodilation, increased vascular permeability & smooth muscle contraction
- These mediators will promote inflammation.
Late phase reactions
- TNFα will activate endothelium at the site, causing expression of adhesion molecules that promote the migration of leucocytes from blood.
- Chemotactic factors are also produced.
- Result in the recruitment of eosinophils, basophils, neutrophils and T cells to the site ~ produce further inflammation.
Methods of diagnosis
- History taking for determining the allergen involved
- Skin tests: Intradermal injection of battery of different allergens
- A wheal and flare (erythema) develop at the site of allergen to which the person is allergic
- Determination of total serum IgE level
- Determination of specific IgE levels to the different allergens
Management
- Avoidance of specific allergen responsible for condition
- Hyposensitization: Injection of gradually increasing doses of extract of allergen
- production of IgG blocking antibody which binds allergen and prevent combination with IgE
- It may induce T cell tolerance
- Drug Therapy: corticosteroidsinjection, epinephrine, antihistamines
7. Type II hypersensitivity
Type II hypersensitivity is also known as cytotoxic hypersensitivity and may affect a variety of organs and tissues.
The reaction time is minutes to hours. Type II hypersensitivity is primarily mediated by antibodies of the IgM or IgG classes and complement.
An antibody (IgG or IgM) reacts with antigen on the cell surface.
This antigen may be part of cell membrane or circulating antigen (or hapten) that attaches to cell membrane.
Clinical conditions
- Transfusion reaction due to ABO incompatibility
- Rh - incompatability (Haemolytic disease of the newborn)
- Autoimmune diseases:
- The mechanism of tissue damage is cytotoxic reactions
- e.g. SLE, autoimmune haemolytic anaemia, idiopathic thrombocytopenic purpura, myasthenia gravis, nephrotoxic nephritis, Hashimoto’s thyroiditis
- A non-cytotoxic Type II hypersensitivity is Grave’s disease:
- It is a form of thyroiditis in which antibodies are produced against TSH surface receptor
- This lead to mimic the effect of TSH and stimulate cells to over-produce thyroid hormones (hyperthyroidism)
- Graft rejection cytotoxic reactions:
- In hyperacute rejection the recipient already has pre-formed antibody against the graft
- Drug reaction:
- Penicillin may attach as haptens to RBCs and induce antibodies which are cytotoxic for the cell-drug complex leading to haemolysis
- Quinine may attach to platelets and the antibodies cause platelets destruction and thrombocytopenic purpura
Diagnostic tests
- Detection of circulating antibody against tissues involved.
- Presence of antibody and complement in the lesion.
- (Biopsy) by immunofluorescence.
8. Type III hypersensitivity
Type III hypersensitivity results when allergens are distributed throughout the body.
The body produces antibodies, which form insoluble antibody-antigen complexes.
The body is unable to clear these, and a large inflammatory response develops.
The antigen is initially soluble and not attached to the organ involved.
Primary components that lead to type III hypersensitivity are insoluble immune complexes and complement. The lesion in the damaged area contains primarily neutrophils (phagocyte) and deposits of immune complexes and complement.
Examples of such allergies include farmer's lung, which is caused by the inhalation of mould growing on hay, and mushroom grower's lung, caused by inhaling the spores produced by mushrooms.
A number of microorganisms can trigger immune complexes.
Streptococcal throat infection may be exacerbated by the formation of these immune complexes, as can the organisms that cause malaria, syphilis and leprosy. Drugs can also have the same effect.
These responses are also involved in autoimmune disorders, when the body's defences attack host tissue. Examples are systemic lupus erythematous (SLE) and rheumatoid arthritis.
Diagnosis
- Tissue biopsies for deposits of immunoglobulins and complement by immunofluorescence.
- Presence of immune complexes in serum and depletion in complement.
- Treatment: anti-inflammatory agents.
9. Type IV hypersensitivity
Type IV reactions are known as delayed hypersensitivities. They appear much more slowly and are caused by the actions of a number of white blood cells. The main effects are caused by a class of immune cell called T-cells.
Inflammatory responses are caused by the release of chemicals from T-cells called Iymphokines.
A well-known manifestation of a Type IV reaction is allergic contact dermatitis. This results from skin contact with, for example, poison ivy, heavy metals (such as lead and mercury), cosmetics and deodorants.
These substances are often too small to evoke an immune response, but upon absorption through the skin, they bind to body proteins and become recognized as 'foreign’.
Diagnostic tests
- Diagnostic tests in vivo include: delayed cutaneous reaction (e.g. Mantoux test) and patch test (for contact dermatitis).
- In vitro tests for delayed hypersensitivity include: mitogenic response, lympho-cytotoxicity and IL-2 production
- Treatment: Corticosteroids and other immunosuppressive agents.
10. Skin prick test
Used to measure allergic responses.
The immunologist limits allergic reactions to local areas of the skin, by pricking the skin with a small drop of allergen.
A 'weal' indicates an allergic response - this is verified by pricking samples of saline and histamine as controls (these should give negative and positive results respectively).
On discovering an allergy, the patient can then take steps to avoid the allergen or, in some cases, undergo desensitizing injections, which exposes the patient to increasing concentrations of allergen.
Skin prick tests may be carried out on the patient's arm or back. Drops of soluble substances, containing common allergens, are placed on the skin. The drop is then pricked into the skin, without drawing blood.
The extent of the allergic response can be quantified by measuring the weal, where local inflammation has occurred. With the information gained, the patient can then take steps to avoid the allergen or possibly undergo desensitizing injections.
009 Tolerance
1. The importance of immune regulation
To avoid excessive lymphocyte activation and tissue damage during normal protective responses against infections
To prevent inappropriate reactions against self antigens (“self-tolerance”)
Failure of control mechanisms is the underlying cause of immune-mediated inflammatory diseases
2. General principles of controlling immune responses
Responses against pathogens decline as the infection is eliminated
–Apoptosis of lymphocytes that lose their survival signals (antigen, etc.)
–Memory cells are the survivors
Active control mechanisms may function to limit responses to persistent antigens (self antigens, possibly tumors and some chronic infections)
–Often grouped under “TOLERANCE”
3. Basic facts
Definition of tolerance: a state of unresponsiveness specific for a given antigen
It is specific (negative) immune response
It is induced by prior exposure to that
antigen
Self tolerance – prevents the body to elicit an immune attack against its own tissues (normal)
Mechanisms of active tolerance prevent inflammatory reactions to many innocuous airborne and food antigens found at mucosal surfaces
• Significance:
–All individuals are tolerant of their own antigens (self-tolerance); breakdown of self-tolerance results in autoimmunity
–Therapeutic potential: Inducing tolerance may be exploited to prevent graft rejection, treat autoimmune and allergic diseases, and prevent immune responses in gene therapy
4. Features of self-tolerance
Self-non-self discrimination is learned during development
Tolerance is NOT genetically programmed
The time of first encounter is critical in determining responsiveness
5. Factors important in the induction of tolerance
- The stage of differentiation of lymphocytes at the time of antigen confrontation
- The site of encounter
- The nature of cells presenting antigenic epitopes
- The number of lymphocytes able to respond
- Microenvironment of encounter (expression of cell adhesion molecules, influence of cytokines etc.)
6. General properties of tolerance
- Immature or developing lymphocyte is more susceptible to tolerance induction than mature one
- Tolerance to foreign antigens is induced even in mature lymphocytes under special conditions
- Tolerance of T lymphocytes is a particularly effective for maintaining long-lived unresponsiveness to self antigens
7. Possible ways of prevention of self-reactivity
Clonal deletion – physical elimination of cells from the repertoire during their lifespan
Clonal anergy – down regulating the intrinsic mechanism of the immune response such as lack of co-stimulatory molecules or insufficient signal for immune cell activation
Suppression – inhibition of cellular activation by interaction with other cells: (e.g. Regulatory T lymphocytes)
8. Division of tolerance
Central
- The site for T cells is the thymus
- The site for B cells is the bone marrow
- The mechanism – clonal deletion
Peripheral
- The site – everywhere in the body
- Cells – both T and B
- Mechanisms –anergy or cell death
The principal fate of lymphocytes that recognize self antigens in the generative organs is death (deletion), BUT:
Some B cells may change their specificity (called “receptor editing”)
Some CD4 T cells may differentiate into regulatory (suppressive) T lymphocytes
9. Immunologically privileged sites
Sites in the body where foreign antigens or tissue grafts do not elicit immune responses.
These antigens do interact with T cells, but instead of destructive immune response they induce tolerance or a response innocent to the tissue.
Immunosuppressive cytokines called TGF-beta (TGF-β) seem to be responsible for such unusual response.
The sites include: brain, eye, testis, uterus (fetus).
10. Ignorance of self antigens
It is a passive form of immunological tolerance.
It occurs when:
- T cells cannot contact with self–antigens
- if self antigen is present in too low an amount to be detected
- if it is present on cells with few or no MHC molecules
- if there are not enough T cells to respond
- if there is the absence of co-stimulation
11. Features of B cell tolerance
- Binding soluble self antigens is tolerogenic for B cells
- Immature B cells that encounter self antigen undergo clonal abortion
- The fate of self-reactive B cells depends on the affinity of the B cell antigen receptor and the nature of the antigen
- Receptor editing - autoreactive B cells escape anergy or deletion by further rearranging their immunoglobulin genes
12. Future applications of tolerance
To promote tolerance to foreign tissue grafts
To control the damaging immune responses in hypersensitivity states and autoimmune diseases
13. Immune mediated inflammatory diseases
Chronic diseases with prominent inflammation, often caused by failure of tolerance or regulation
–RA, IBD, MS, psoriasis, many others
–Affect 2-5% of people, incidence increasing
May result from immune responses against self antigens (autoimmunity) or microbial antigens (Crohn’s disease?)
May be caused by T cells and antibodies
May be systemic or organ-specific
Immune-mediated inflammatory diseases develop because the normal controls on immune responses fail
The phenotype of the disease is determined by the nature of the immune
response
These diseases often become self-perpetuating
Experimental models are revealing pathways of immune regulation and why it fails
Genetic studies are identifying underlying defects in human diseases
Improving technologies are enabling analyses of patients
Challenges:
–From genes to pathways (molecular and functional)
–Using the knowledge to develop therapies
14. Features of autoimmune
diseases
Fundamental problem: imbalance between immune activation and control
–Underlying causative factors: susceptibility genes + environmental influences
–Immune response is inappropriately directed or controlled; effector mechanisms of injury are the same as in normal responses to microbes
Nature of disease is determined by the type of dominant immune response
Many immunological diseases are chronic and self-perpetuating
15. Pathogenesis of autoimmunity
Susceptibility genes
Failure of self-tolerance
Persistence of functional self-reactive lymphocytes
–Environmental trigger (e.g. infections,
tissue injury)
Activation of self-reactive lymphocytes
Immune responses against self tissues
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