Immune Response Past Paper PDF

Unit II ✓ Primary and secondary immune response: ✓ Antigens, ✓ Immunogens, ✓ Adjuvant ✓ Haptens, ✓ Immunoglobulins – Structure, classes and subclasses. ✓ Hyper variable region, ✓ isotypic, allotypic and idiotypic variation, ✓ Subsets of T cells and B cells, T-helper cells, T-killer cells, suppressor...

Unit II ✓ Primary and secondary immune response: ✓ Antigens, ✓ Immunogens, ✓ Adjuvant ✓ Haptens, ✓ Immunoglobulins – Structure, classes and subclasses. ✓ Hyper variable region, ✓ isotypic, allotypic and idiotypic variation, ✓ Subsets of T cells and B cells, T-helper cells, T-killer cells, suppressor cells. Development of T and B cells. ✓ T and B cell receptors, antigen processing and presentation. ✓ T and B cell interaction. ✓ Cytokines and co-stimulatory molecules - Lymphokines, interleukins structure and function, ✓ complement system. Primary and secondary humoral immune responses. In a primary immune response, ✓ naive B cells are stimulated by antigen, become activated, and differentiate into antibody- secreting cells that produce antibodies specific for the eliciting antigen. ✓ A secondary immune response is elicited when the same antigen stimulates memory B cells, leading to production of greater quantities of specific antibody than are produced in the primary response. ✓ Note that the characteristics of secondary antibody responses summarized in the table are ✓ typical of T-dependent antibody responses to protein antigens. Primary immune response ✓An adaptive immune response that occurs after the first exposure of an individual to a foreign antigen. ✓Primary responses are characterized by relatively slow kinetics and small magnitude compared with secondary (memory) responses after a second or subsequent exposure. ✓Secondary immune response : An adaptive immune response that occurs on second exposure to an antigen. ✓A secondary response is characterized by more rapid kinetics and greater magnitude relative to the primary immune response, which occurs on first exposure. ✓Exposure of the immune system to a foreign antigen enhances its ability to respond again to that antigen. ✓Responses to second and subsequent exposures to the same antigen, called secondary immune responses, are usually more rapid, greater in magnitude, and often qualitatively different from the first, or primary, immune response to that antigen. ✓Plasma or serum proteins can be physically separated based on solubility characteristics into albumins and globulins and may be more precisely separated, based on differences in charge, using a technique called electrophoresis. ✓ In electrophoretic separations of serum or plasma, most antibodies are found in the third- fastest migrating group of globulins, named gamma globulins for the third letter of the Greek alphabet. ✓(Note that gamma globulins include all classes of antibodies, described later, not just the IgG class.) ✓Another common name for antibody is immunoglobulin (Ig), referring to the immunity- conferring portion of the globulin fraction of serum or plasma. Antigen : Any substance (usually foreign) that binds specifically to an antibody or a T-cell receptor; often is used as a synonym for immunogen. Antigen processing : Degradation of antigens by one of two pathways yielding antigenic peptides that are displayed bound to MHC molecules on the surface of antigen-presenting cells or altered self cells. Antigen-presenting cell (APC) : ✓Any cell that can process and present antigenic peptides in association with class II MHC molecules and deliver a costimulatory signal necessary for T-cell activation. Example: Macrophages, dendritic cells, and B cells constitute the professional APCs. ✓ Nonprofessional APCs, which function in antigen presentation only for short periods include thymic epithelial cells and vascular endothelial cells.  Class 1 and class 2 MHC ✓mature T cells can be subdivided into two populations according to their expression of CD4 or CD8 on the plasma membrane. ✓CD4+ T cells recognize peptides that are combined with class II MHC molecules, and function primarily as helper or regulatory T cells, whereas CD8+ T cells recognize antigen that is expressed on the surface of class I MHC molecules, and function mainly as cytotoxic T cells. Class I major histocompatibility complex (MHC) molecule ✓One of two forms of polymorphic heterodimeric membrane proteins ✓that bind and display peptide fragments of protein antigens on the surface of APCs for recognition by T lymphocytes. ✓Class I MHC molecules usually display peptides derived from proteins in the cytosol of the cell, for recognition by CD8+ T cells. Class II major histocompatibility complex (MHC) molecule ✓ One of two major classes of polymorphic heterodimeric membrane proteins ✓that bind and display peptide fragments of protein antigens on the surface of APCs for recognition by T lymphocytes. ✓Class II MHC molecules usually display peptides derived from extracellular proteins that are internalized into phagocytic or endocytic vesicles, for recognition by CD4+ T cells. Adjuvants ✓ bacterial components or other substances, ✓ typically suspended in a medium such as oil that prolongs their dispersal into the tissues, ✓ administered together with vaccines to heighten the effectiveness of the vaccination. ✓ The bacterial (or other) material provokes a mild inflammation that attracts phagocytes and accelerates their activation and antigen presentation to T cells for development of specific adaptive immune responses. ✓ Some vaccine components themselves can serve as adjuvants. The pertussis component (from Bordetella pertussis) in DTP (Diphtheria- Tetanus Pertussis) vaccine is also an effective adjuvant. ✓ Other adjuvants include alum and BCG (Bacillus Calmette Guerin). ✓ BCG includes material derived from Mycobacterium and is in wide use around the world as a vaccine against tuberculosis, particularly in areas of high incidence. ✓ Its use has declined in some areas where the incidence of tuberculosis has significantly declined. ✓ In the United States (and several other countries), BCG is not used routinely for human vaccinations because it interferes with the use of skin testing (creating false positives) in tuberculosis studies and because of adverse reactions (e.g., disseminated BCG infection). ✓ However, BCG is still used in the United States for certain high-risk individuals or populations. Hapten : ✓A small chemical that can bind to an antibody but must be attached to a macromolecule (carrier) to stimulate an adaptive immune response specific for that chemical. ✓For example, immunization with dinitrophenol (DNP) alone will not stimulate an anti-DNP antibody response, but immunization with a protein with covalently attached DNP hapten will. ✓Haptens are small, ✓normally nonimmunogenic, molecules, ✓ usually of nonbiologic origin, ✓that behave like synthetic epitopes. ✓Haptens are antigens and can bind to immune receptors but cannot by themselves induce a specific immune response and hence are not immunogenic. ✓However, when a hapten is chemically bound to an immunogen (also called a carrier), ✓ immune responses may be generated against both the hapten and the epitopes on the immunogen. Tolerogens ✓ During development of the immune repertoire (the sum of all of the epitopes for which a given individual has generated immunologic receptors), tolerance to self molecules and cells develops first. ✓ a lack of immune response to self antigens exists in the normal, healthy state. ✓ Nonself antigens are subsequently recognized as foreign. ✓ Tolerance can also develop later in life, for example to antigens that are administered orally. ✓ Tolerogens induce adaptive immune unresponsiveness. ✓ However, unlike immunogens, exposure to a tolerogen results in a diminished response rather than an enhanced one. Immunoglobulins ✓ Immunoglobulins are synthesized by B lymphocytes (B cells) and are both synthesized and secreted by plasma cells. ✓ Plasma cells are B cells that have terminally differentiated. ✓ The term antibody is applied to an immunoglobulin molecule with specificity for an epitope of the molecules that make up antigens. ✓ Antibodies non-co-valently bind to antigens to immobilize them, render them harmless, or “tag” the antigen for destruction and removal by other components of the immune system. ✓ In doing so, antibodies facilitate the ability of other cells and molecules in the immune system to identify and interact with antigens. ✓ Because antibodies are often in soluble form, they are important components of humoral (soluble) immune responses. Immunoglobulin monomer. An immunoglobulin monomer contains 2 identical light (L) chains and two identical heavy (H) chains connected by disulfide bonds. ✓ Each chain contains a variable domain and one or more constant domains Basic structure ✓Human immunoglobulin contains four polypeptides: two identical light chains and two identical heavy chains linked by disulfide bonds to form a monomeric unit. ✓Heavy and light chains are aligned such that the amino portion (NH terminus) of a single heavy and a single light chain form an epitope-binding site. ✓Each heavy and light chain may be subdivided into homologous regions termed domains. ✓Light chains, termed K (kappa) or λ (lambda), are encoded on chromosomes 2 and 22, respectively. ✓There are 5 types of heavy chains, all encoded on chromosome 14, termed mu (μ ), delta (δ), gamma (γ), ep-silon (Ɛ), and alpha (α). ✓ The genetically different forms of light chains (K and λ) and of heavy chains (μ,δ,Ɛ,α,γ) are known as iso-types. Immunoglobulin class or subclass is determined by the heavy chain isotype. ✓ Immunoglobulin domains. Light chains are of two types (κ and λ) while there are five types of heavy chains (α, δ, ε, γ, µ). ✓ Immunoglobulin light and heavy changes are divisible into domains that consist of approximately 110 amino acids and contain an interchange disulfide bond (VL = light chain variable domain, VH = heavy chain variable domain, CL = light chain constant domain, CH = heavy chain constant domain) 1. Light chains : ✓ An immunoglobulin monomer contains two identical K or two identical λ light chains but never one of each. ✓ Light or L chains contain a variable (VL) domain and a constant (CL) domain. ✓ Each domain contains about 110 amino acids and an intrachain disulfide bond. ✓Variable regions (in both heavy and light chains) are so named for their variation in amino acid sequences between immunoglobulins synthesized by different B cells. 2. Heavy chains : Heavy chains contain one variable (VH) and three or four constant (CH) domains. (1V, 3 C.D) ✓ Heavy (H) chain variable domains (VH) are extremely diverse, and constant domains (CH) display a relatively limited variability for members of an isotype. ✓ The δ, Ƴ, and α heavy chains contain 3 constant domains (CH1, CH2, CH3), and μ and ε heavy chains contain a 4 constant domain (CH4), making them both longer and heavier than δ, Y, or α heavy chains. δ- CH1, Ƴ- CH2 ,α- CH3. μ- CH4, ε- CH4 3. Antigen-binding sites : A light chain variable domain and a heavy chain variable domain together form a pocket that constitutes the antigen (epitope)-binding region of the immunoglobulin molecule. Because an immunoglobulin monomer contains two identical light chains and two identical heavy chains, the two binding sites found in each monomeric immunoglobulin are also identical. The variability in the amino acid sequences of the VL and VH domains, together with the random pairing of light and heavy chain that occurs from one B cell to another, creates a pool of binding sites capable of recognizing a very large number of different epitopes. 4. Immunoglobulin: ✓ landmarks Immunoglobulin molecules can be enzymatically cleaved into discrete fragments by either pepsin or papain. ✓ Disulfide bonds join the heavy chains at or near a proline-rich hinge region, which confers flexibility on the immunoglobulin molecule. The fragments of immunoglobulin are as follows ✓Fab or antigen (epitope)-binding fragment, produced by papain cleavage, ✓contains VH, CH1, VL, and CL. ✓2 Fab fragments are produced by papain cleavage of an immunoglobulin monomer; each fragment has an epitope-binding site. ✓Fc or constant (crystallizable) fragment is produced by papain cleavage. ✓The Fc portion contains the CH2, CH3, and (sometimes) CH4 regions of the immunoglobulin molecule. ✓It is responsible for many biologic activities that occur following engagement of an epitope. ✓Fd is the heavy chain (VH, CH1) portion of Fab. ✓Fd' is a heavy chain (VH, CH1) portion of Fab. ✓The prime (′) mark denotes extra amino acids due to a pepsin cleavage site. ✓F(ab')2 is a dimeric molecule produced by pepsin cleavage. ✓An immunoglobulin monomer will produce a single F(ab')2 fragment containing two (VH, CH1′) segments joined by disulfide bonds. ✓An F(ab')2 contains two epitope-binding sites. ✓Enzyme cleavage of immunoglobulin determines landmarks. ✓Papain cleaves heavy chains to form two identical Fab fragments (each containing one binding site) and one Fc fragment. ✓Pepsin cleaves heavy chains at a point that produces an F(ab')2 fragment containing two linked binding sites and remaining heavy chain material that is degraded and eliminated ✓Isotypes : ✓Heavy chain isotypes (µ, δ, γ, α, and ε) also determine immunoglobulin isotype or class (IgM, IgD, IgG, IgA, and IgE, respectively). ✓Normally humans produce all 5 immunoglobulin isotypes. ✓Of the 2 light chain isotypes, an individual B cell will produce only κ or λ chains, never both. ✓B cells express surface-bound immunoglobulin monomers as epitope-specific receptors; ✓B cells produce and display only one isotype, with the exception that unstimulated B cells express both IgM and IgD. ✓When secreted into the body fluids, soluble IgG and IgE remain monomeric, ✓ soluble IgM forms a pentamer, and ✓ soluble IgA can be found in either a monomeric or dimeric form. ✓ IgM is found either as a cell-surface-bound monomer (2µ + 2κ or 2λ) or as a secreted pentamer with 10 H and L chains linked by disulfide bonds and a J (“joining”) chain [five monomers + J, i.e., 5 x (2µ + 2κ or 2λ) + J]. ✓ Most B cells display IgM on their cell surfaces. ✓ In general, IgM is the first immunoglobulin to be formed following antigenic stimulation. ✓ IgM is effective both at immobilizing antigen (agglutination;) and in activating the classical pathway of complement. ✓ IgD has a monomeric structure (2δ + 2κ or 2λ) and is almost exclusively displayed on B cell surfaces. ✓ Little is known of its function. ✓ IgG exists as both surface and secreted monomeric (2γ + 2κ or 2λ) molecules. ✓ Four subclasses (γ1, γ2, γ3, and γ4) of γ heavy chains account for the four human IgG subclasses, IgG1, IgG2, IgG3, and IgG4. ✓ Collectively, IgG subclasses make up the greatest amount of immunoglobulin in the serum. ✓ Many IgG antibodies are effective in activating complement , opsonizing and neutralizing microorganisms and viruses, and initiating antibody-dependent cell- mediated cytotoxicity, and they function in a wide variety of hypersensitivity functions. ✓ IgA is present in both monomeric and dimeric forms. ✓ Monomeric IgA (2α + 2κ or 2λ) is found in the serum. ✓ The addition of a J or joining chain to two IgA monomers forms a dimer. ✓ Epithelial cells use a specialized receptor to transport the IgA dimer to mucosal surfaces. ✓ This specialized receptor becomes an accessory molecule. Hypervariable regions of an antibody ✓The hypervariable regions of an antibody, also known as complementarity-determining regions (CDRs),are crucial for the antibody’s ability to bind to specific antigens. ✓These regions are found in both the light and heavy chains of the antibody and are highly variable in their amino acid sequences. ✓This variability allows antibodies to recognize and bind to a vast array of antigens with high specificity. ✓Each antibody has three hypervariable regions in its variable domain, which come together to form the antigen- binding site. ✓These regions directly interact with the antigen, determining the antibody’s specificity. Isotypes, Allotypes, and Idiotype antibodies : ✓Human antibodies are Y-shaped, tetrapeptide glycoproteins made by two heavy chains and two light chains that are bound together by disulfide bonds. ✓Like any other proteins, antibodies can act as an antigen, if injected into different species or hosts. For example, if antibodies from humans are injected into mice, mice will recognize these antibodies are foreign proteins (antigens) and will form antibodies against human antibodies (i.e. antihuman antibodies). ✓ It is observed that the entire immunoglobulin is not immunogenic but it contains antigenic determinants at specific sites. ✓ Based on the location of those antigenic determinants, immunoglobulins are divided into, isotypes, allotypes, and idiotypes. Isotypes ✓ Each antibody has only one type of (γ, or α, or μ, or ε, or δ) heavy chain and one type of (k or λ) light chain. ✓ The structural differences in the constant region of a heavy chain or light chain determine immunoglobulin (Ig) class and sub-class, types and subtypes within a species. ✓ These constant region determinants are called isotypic determinants or isotypes. ✓ Formation of anti-isotypic antibody : All members of a species carry the same constant-region genes (including multiple alleles) so when an antibody from one species is injected into another species, the isotypic determinants will be recognized as foreign, forming anti-isotypic antibody. ✓ Although all members of a species inherit the same set of isotype genes, multiple alleles exist for some of the genes. ✓ These alleles encode subtle amino acid differences. ✓ Products of allelic forms of the same gene will have slightly different amino acid sequences in the constant regions, which are known as allotypic determinants. ✓ The sum of the individual allotypic determinants displayed by an antibody determines its allotype. Rhesus Antigen ✓The Rhesus (Rh) antigens, are another clinically important set of blood group antigens. Rh antigens are nonglycosylated, hydrophobic cell surface proteins found in red blood cell membranes and are structurally related to other red blood cell membrane proteins with transporter functions. ✓Rh proteins are encoded by two tightly linked and highly homologous genes, but only one of them, called RhD, is commonly considered in clinical blood typing. ✓ This is because up to 15% of the population has a deletion or other alteration of the RhD allele. ✓These people, called Rh negative, are not tolerant to the RhD antigen and will make antibodies to the antigen if they are exposed to Rh-positive blood cells. ✓The major clinical significance of anti-Rh antibodies is related to hemolytic reactions in developing fetuses that are similar to transfusion reactions. ✓ Rh-negative mothers carrying an Rh-positive fetus can be sensitized by fetal red blood cells that enter the maternal circulation, usually during childbirth. ✓Because the Rh antigen is a protein, as opposed to the carbohydrate ABO antigens, class-switched high-affinity IgG antibodies specific for Rh are produced in Rh-negative mothers. ✓Subsequent pregnancies in which the fetus is Rh positive are at risk because the maternal anti-Rh IgG antibodies can cross the placenta and mediate the destruction of the fetal red blood cells. ✓ This causes hemolytic disease of the fetus and newborn (also called erythroblastosis fetalis) and can be lethal for the fetus. ✓This disease can be prevented by administration of anti-RhD antibodies to the mother within 72 hours of birth of the first Rh-positive baby. ✓The treatment prevents the baby’s Rhpositive red blood cells that entered the mother’s circulation from inducing the production of anti-Rh antibodies in the mother. ✓The exact mechanisms of action of the administered antibodies are not clear but may include phagocytic clearance or complement-mediated lysis of the baby’s red blood cells before they can elicit an antibody response in the mother, or Fc receptor–dependent feedback inhibition of the mother’s RhD-specific B cells. Allotypes in human ✓ Allotypes have been characterized for all four IgG subclasses, IgA2 subclass, and Kappa ✓ light chain. ✓ They are designated by the class and subclass followed by the allele number. ✓ 1. Gamma-chain allotypes (also known as Gm markers): So far at least 25 different Gm ✓ allotypes have been identified e.g. G1m(1), G2m(23), G3m (11), G4m(4a). ✓ 2. IgA2 subclass has 2 allotypes designated as A2m(1) and A2m(2). ✓ 3. Kappa (κ) light chain has 3 allotypes, designated Km(1), Km(2), and Km(3). ✓ Each of these allotypic determinants represents differences in one to four amino acids that are encoded by different alleles. Formation of anti-allotypic antibodies ✓ Antibody to allotypic determinants can be produced by injecting antibodies from one member of a species into another member of the same species who carries different allotypic determinants. ✓ Sometimes, a pregnant mother can produce antibodies to paternal allotypic determinants present on the fetal immunoglobulin. ✓ Anti-allotype antibodies may also be developed following blood transfusion. Idiotype ✓ VH and VL domains of an antibody constitute an antigen-binding site. ✓ To recognize the vast array of antigens that a human can encounter in its lifetime, this variable region has different structural conformation owing to the presence of different amino acids. ✓ There are millions of such antibodies in the human body specific for each antigen. ✓ These unique amino acid sequences present in the VH and VL domains of a given antibody also serves as a set of antigenic determinants. ✓ Each individual antigenic determinant of the variable region is referred to as an idiotype. Each antibody will present multiple idiotopes; the sum of the individual idiotopes is called the idiotype of the antibody. ✓ Formation of anti-idiotype antibodies A single clone of plasma cells produces immunoglobulin molecules with identical variable region sequences i.e., they all have the same idiotype. ✓ When antibodies having no or minimal variation in their isotypes and allotypes are injected into a genetically identical person, anti-idiotype antibodies will be formed. Subsets of T Lymphocytes (6 types of T cells) ✓ The two major T cell subsets are defined by the cell surface expression of the CD4 and CD8 proteins. ✓ T cells are the mediators of cellular immunity: CD4+ T cells are helper T lymphocytes or their naive precursors, and CD8+ T cells are CTLs or their precursors. ✓ Both CD4+ and CD8+ T cells express antigen receptors called αβ T cell receptors (TCRs). ✓ CD4+ helper T cells secrete cytokines that act on various other cells, including other T lymphocytes, B cells, and macrophages. ✓ CD8+ CTLs recognize and kill cells infected with viruses and other microbes that can live inside host cells, and also kill cancer cells. ✓ CD4+ regulatory T cells are a third subset of T cells expressing αβ receptors; their function is to inhibit immune responses. ✓ natural killer T (NKT) cells, mucosa associated invariant T (MAIT) cells, and γδ T cells are 3 numerically smaller subsets of T cells that express TCRs with limited diversity, analogous to the antibodies made by B-1 cells. TGF-β regulates the differentiation of functionally distinct subsets of T cells. subsets of T helper cells (Th1, Th2, Th17, Treg) ✓invariant NKT (iNKT) cells, express αβ T cell antigen receptors with very little diversity and recognize lipid antigens, that are bound to class I MHC–like molecules called CD1 molecules. ✓All NKT cell TCRs recognize lipids that are bound to class I MHC–like molecules called CD1 molecules. ✓There are several CD1 proteins expressed in humans and mice.  B cells develop into follicular, marginal zone, and B-1 cells T-dependent and T-independent antibody responses-dependent antibody responses to protein antigens mainly involve follicular B cells. T-independent responses to multivalent antigens are mediated mainly by marginal zone B cells in the spleen and B-1 cells in mucosal sites. Ig, Immunoglobulin. Bone marrow–derived HSCs give rise to the majority of circulating B cells (follicular B cells) and a subset of B cells called marginal zone B cells. 1.Marginal zone B lymphocytes : A subset of B lymphocytes, found in the marginal zone of the spleen, that respond rapidly to blood-borne microbial antigens by producing IgM antibodies with limited diversity. whereas marginal zone B cells are present only in the spleen in rodents but can be found in the circulation of humans. 2.B-1 lymphocytes : A subset of B lymphocytes that develop earlier during ontogeny than do follicular B cells, express a limited repertoire of V genes with little junctional diversity, and secrete IgM antibodies in response to T independent antigens. B-1 cells are located mainly in mucosal tissues and the peritoneal and pleural cavities. 3.Follicular B cells : give rise to most of the high-affinity antibodies and memory B cells that protect people from repeat infections by the same microbes. In contrast, B-1 and marginal zone B cells make up a minority of B cells and produce antibodies with limited diversity., Subsets of B Cells That Respond to T-Independent Antigens The major subsets of B cells are follicular B cells, marginal zone B cells, and B-1 cells, each of which is found in distinct anatomic locations within lymphoid tissues.. ✓ The marginal zone and B-1 subsets of B cells are especially important for antibody responses to TI antigens. ✓ Whereas responses to T-dependent protein antigens are largely mediated by follicular B cells, other B cell subsets may be the primary responders to TI antigens. ✓ Marginal zone B cells are a distinct population of B cells that mainly respond to polysaccharides. ✓ After activation, these cells differentiate into short-lived plasma cells that produce mainly IgM. ✓ B-1 cells represent another lineage of B cells that responds readily to TI antigens mainly in the peritoneum and in mucosal sites. ✓ T-independent antibody responses are initiated mainly in the spleen, peritoneal cavity, and mucosal sites. ✓ Macrophages located in the marginal zones surrounding lymphoid follicles in the spleen are particularly efficient at trapping polysaccharides when these antigens are injected intravenously. ✓ TI antigens may persist for prolonged periods on the surfaces of marginal zone macrophages, where they are recognized by specific B cells. Development of Th1 Cells : Th1 differentiation is driven mainly by the cytokines IL-12 and IFN-γ and occurs in response to microbes that activate DCs, macrophages and NK cells ✓ Development of Th1 cells. ✓Interleukin-12 (IL-12) produced by dendritic cells and macrophages in response to microbes, including intracellular microbes, and ✓ interferon-γ (IFN-γ) produced by natural killer (NK) cells (all part of the early innate immune response to the microbes) activate the transcription factors T- BET, STAT1, and STAT4, which stimulate the differentiation of naive CD4+ T cells to the Th1 subset. ✓IFN-γ produced by the Th1 cells amplifies this response and inhibits the development of Th2 and Th17 cells. ✓Other cytokines, including type I IFNs and IL-18, also promote Th1 differentiation but are not shown here. Development of Th2 Cells : ✓ Th2 differentiation occurs in response to helminths and allergens and is dependent on the cytokine IL-4. ✓IL-4 stimulates the development of Th2 effector cells from naive CD4 + T cells and may be a growth factor for differentiated Th2 cells. ✓Interleukin-4 IL-4 is the signature cytokine of the Th2 subset and functions as both an inducer and an effector cytokine of these cells. ✓Development of Th2 cells: ✓Dendritic cells may respond to cytokines produced in epithelia by becoming Th2 inducers, by mechanisms that are not well defined. ✓Interleukin-4 (IL-4) produced by activated T cells themselves or by mast cells and eosinophils, especially in response to helminths, activates the transcription factors GATA3 and STAT6, which stimulate the differentiation of naive CD4+ T cells to the Th2 subset. ✓IL-4 produced by the Th2 cells amplifies this response and inhibits the development of Th1 and Th17 cells. ✓TSLP, Thymic stromal lymphopoietin.  Multipotent stem cells give rise to distinct B and T lineages. ✓Hematopoietic stem cells (HSCs) give rise to distinct progenitors for various types of blood cells. ✓One of these progenitor populations (shown here) is called a common lymphoid progenitor (CLP). ✓Pro-B cells can eventually differentiate into follicular (FO) B cells, marginal zone (MZ) B cells, and B-1 cells. ✓ Pro-T cells may commit to either the αβ or γδ T cell lineages. ✓ Commitment to different lineages is driven by various transcription factors, indicated in italics. ✓The major subsets of B cells are follicular B cells, marginal zone B cells, and B-1 cells, each of which is found in distinct anatomic locations within lymphoid tissues. Follicular B cells, the most numerous type of B cells in the body, are found in lymphoid tissues and blood. ✓They express highly diverse, surface antigen receptors as the key secreted effector molecules of adaptive humoral immunity ✓ EARLY FACTOR 2- E2A ✓During B and T cell development, committed progenitor cells proliferate ✓ EARLY B CELL FACTOR-EBF ✓ first in response to cytokines and ✓ PAIRED BOX GENE2 –POX2 ✓ later in response to signals generated by a pre-antigen receptor that ✓ Id2 –DNA binding protein inhibitor 2 select cells that have successfully rearranged the first set of antigen ✓ GATA 3- receptor genes. ✓ Notch1 ✓ Numerous transcription factors are involved in the maturation of T and B cells ✓ Notch1 and GATA3 commit developing lymphocytes to the T cell lineage. ✓ The Notch family of proteins are cell surface molecules that are proteolytically cleaved when they interact with specific ligands on neighboring cells. ✓ The cleaved intracellular portions of Notch proteins migrate to the nucleus and modulate the expression of specific target genes. ✓ Notch1 is activated in lymphoid progenitor cells, and together with GATA3 it induces expression of a number of genes that are required for the further development of αβ T cells. ✓ The EBF, E2A, and PAX5 transcription factors induce the expression of genes required for B cell development. Selected members of the immune receptor family. ✓ 5 selected members of the immune receptor family are depicted. ✓ Typically, immune receptors that activate immune cells have separate polypeptide chains for recognition and associated polypeptide chains that contain cytosolic ITAMs. ✓ Examples shown here include the B cell receptor (BCR), the T cell receptor (TCR), and the high affinity receptor for IgE (FcεRI). Inhibitory receptors in the immune system typically have ITIMs on the cytosolic portion of the same chain that uses its extracellular domain for ligand recognition. ✓ FcγRIIB is an inhibitory receptor found on B cells and myeloid cells. ✓ PD-1, an inhibitory receptor found on T cells, also has an immunoreceptor tyrosine-based switch motif (ITSM) in its cytoplasmic tail. ✓ Ig, Immunoglobulin; ITAM, immunoreceptor tyrosine-based activation motif; ITIM, immunoreceptor tyrosine-based inhibition motif.  T cell Receptor (TCR) Immunoglobin (Ig) Components α and β chains (most common form Heavy and light chains of TCR) Number of Ig domains One V domain and one C domain in Heavy chain: One V domain, three each chain or four C domains Light chain: One V domain and one C domain Number of CDRs involved in Six (three in each chain) Six (three in each chain) antigen binding Associated signaling molecules CD3 and ζ Igα and Igβ Affinity for antigen (Kd ) 10 −5–10 −7 M 10 −7–10 −11 M Changes after cellular activation Production of secreted form No Yes Isotype switching No Yes Somatic mutations No Yes ✓ Cell surface molecules involved in the interaction between B and TH cells Membrane immunoglobulin (mIg) takes up antigen (Ag) into an intracellular compartment where it is degraded and peptides can combine with major histocompatibility complex (MHC) class II molecules. ✓ Other arrows show the discrete signal transduction events that have been established. ✓ A and B are the antigen–receptor signal transduction events involving tyrosine phosphorylation and phosphoinositide breakdown. ✓ The antigen receptors also regulate LFA-1 affinity for ICAM-1 and ICAM-3. ✓ CD28 also sends a unique signal to the T cell (C). ✓ In the later phases of the response CTLA-4 can supplant CD28 to cause down regulation. ✓ In the B cell, stimulation via CD40 is the most potent activating signal (D). ✓ In addition, class II MHC molecules appear to induce distinct signaling events (E) Direct interaction of B cells and T cells involves Co-stimulatory molecules. ✓ Direct interaction of B cells and T cells involves Co-stimulatory molecules. ✓ Antigen-specific T-cell populations can be obtained by growing and cloning T cells with antigens, APCs and IL-2. ✓ It is thus possible to visualize directly B-cell and T-cell clusters interacting in vitro: ✓ the T cells become polarized, with the T-cell receptors concentrated on the B-cell side; ✓ the B cells also become polarized and express most of their MHC class II molecules and ICAM-1 in proximity to the T cells. ✓ The interactions in these clusters strongly suggest an intense exchange of information, which leads to two important events in the B-cell life cycle: ✓ induction of proliferation; and ✓ differentiation into plasma cells. ✓ The initial interaction between a naive B cell and a cognate antigen via the BCR in the presence of cytokines or other growth stimuli induces activation and proliferation of the B cell. ✓ This then leads to processing of the T-dependent antigen and presentation to T cells. ✓ The interaction between B cells and T cells is a two-way process in which B cells present antigen to T cells and receive signals from the T cells for division and differentiation ✓ APCs also express costimulatory molecules to optimally activate T lymphocytes. ✓ The prototypic costimulatory receptor is CD28 on T cells, which is activated when bound by the costimulatory molecules B7-1 (CD80) and B7-2 (CD86) expressed on APCs. ✓ The best characterized costimulatory pathway in T cell activation involves the T cell surface receptor CD28, which binds the costimulatory molecules B7-1 (CD80) and B7-2 (CD86) expressed on the surface of activated APCs. ✓Cytokines ✓Cytokines are low-molecular-weight soluble protein messengers that are involved in all aspects of the innate and adaptive immune response, including cellular growth and differentiation, inflammation, and repair. ✓Originally called lymphokines and monokines to reflect lymphocytic or monocytic origin, we now recognize that these substances are produced by a wide variety of leukocytes and nonleuko-cytes. ✓A large number of cytokines have been identified, although the roles of many of them are not yet fully understood (Table 6.2). Many cytokines are crucial in regulating lymphocyte development and in determining the types of immune responses evoked by specific responses ✓Lymphokine An old name for a cytokine (soluble protein mediator of immune responses) produced by lymphocytes Complement A system of plasma and cell surface proteins that interact with one another and with other molecules of the immune system to generate important effectors of innate and adaptive immune responses. The classical, alternative, and lectin pathways of the complement system are activated by antigen- antibody complexes, microbial surfaces, and plasma lectins binding to microbes, respectively, and consist of a cascade of proteolytic enzymes that generate inflammatory mediators and opsonins. All three pathways lead to the formation of a common terminal cell lytic complex that is inserted in cell membranes. Activation of the complement system generates protein molecules or peptide fragments, which have the following effects: 1.Opsonization of microorganisms for uptake by phagocytes and eventual intracellular killing; 2. attraction of phagocytes to sites of infection (chemotaxis); 3. increased blood flow to the site of activation and increased permeability of capillaries to plasma molecules; 4. damage to plasma membranes on cells, Gram-negative bacteria, enveloped viruses, or other organisms that have caused complement activation; 5. release of inflammatory mediators from mast cells. The Complement System The complement system consists of several plasma proteins that work together to opsonize microbes, to promote the recruitment of phagocytes to the site of infection, and in some cases to directly kill the microbes. ✓ The classical pathway, so called because it was discovered first, uses a plasma protein called C1q to detect antibodies bound to the surface of microbes or other structures. ✓ Once C1q binds to the Fc portion of the antibodies, two associated serine proteases, called C1r and C1s, become active and initiate a proteolytic cascade involving other complement proteins. ✓ The classical pathway is one of the major effector mechanisms of the humoral arm of adaptive immune responses. ✓ Innate immune system soluble proteins called pentraxins,, can also bind C1q and initiate the classical pathway. Pathways of complement activation. ✓ The activation of the complement system may be initiated by three distinct pathways, ✓ Recognition of microbes by any of the three complement pathways results in sequential recruitment and assembly of additional complement proteins into protease complexes ✓ all of which lead to the production of C3a-stimulates inflammation, ✓ C3b (early steps)- C3b initiates the late steps of complement activation. ✓ culminating in the production of peptides that also stimulate inflammation (C5a) and polymerized C9,-membrane attack complex (late steps). ✓ One of these complexes, called C3 convertase, cleaves the central protein of the complement system, C3, producing C3a and C3b. ✓ The larger C3b fragment becomes covalently attached to the microbial surface where the complement pathway was activated. ✓ The sequential enzymatic activity of complement proteins provides such tremendous amplification that millions of C3b molecules can deposit on the surface of a microbe within 2 or 3 minutes. ✓ C3b serves as an opsonin to promote phagocytosis of the microbes. ✓ The smaller fragment, C3a, is released and stimulates inflammation by acting as a chemo attractant for neutrophils, by inducing mast cell degranulation, and by directly increasing vascular permeability so that plasma proteins and fluid leak into sites of infections. ✓C3b binds other complement proteins to form a protease called C5 convertase that cleaves C5, generating a released peptide (C5a) and a larger fragment (C5b) that ✓remains attached to the microbial cell membranes. ✓C5a exerts the same proinflammatory effects as C3a and is more potent. ✓C5b initiates the formation of a complex of the complement proteins C6, C7, C8, and C9, which are assembled into a membrane pore called the membrane attack complex (MAC) that causes lysis of the cells where complement is activated. ✓patients with deficiencies in C3 are highly susceptible to recurrent, often lethal, bacterial infections. Anaphylatoxins : The C5a, C4a, and C3a complement fragments that are generated during complement activation. The anaphylatoxins bind specific cell surface receptors and promote acute inflammation by stimulating neutrophil chemotaxis and activating mast cells. Opsonin A molecule that becomes attached to the surface of a microbe and can be recognized by surface receptors of neutrophils and macrophages, thereby increasing the efficiency of phagocytosis of the microbe. Opsonins include IgG antibodies, which are recognized by the Fcγ receptor on phagocytes, and fragments of the C3 complement protein, which are recognized by CR1 (CD35) and by the leukocyte integrin MAC-1. (1)The complement pathways are initiated by proteins that bind to pathogens, either directly or via an antibody or other pathogenspecific protein. After a conformational change, (2)enzymatic mediators activate other enzymes that generate the central proteins of the complement cascade, the C3 and C5 convertases, which cleave C3 and C5, releasing active components that mediate all functions of complement, including (3)opsonization, (4)inflammation, and (5) the generation of the membrane attack complex (MAC). Effector complement proteins can label an antibody-antigen complex for phagocytosis (opsonins), initiate inflammation (anaphylatoxins), or bind to a pathogen and nucleate the formation of the MAC. Often, these effectors act through (6)complement receptors on phagocytic cells, granulocytes, or erythrocytes. (7)Regulatory proteins limit the effects of complement by promoting their degradation or preventing their binding to host cells Collectins A family of proteins, including mannose-binding lectin, that is characterized by a collagen-like domain and a lectin (i.e., carbohydrate binding) domain. Collectins play a role in the innate immune system by acting as microbial pattern recognition receptors, and they may activate the complement system by binding to C1q. Mannose-binding lectin (MBL) A plasma protein that binds to mannose residues on microbial surfaces, thereby initiating the lectin pathway of complement activation. Macrophages express a surface receptor for C1q that can also bind MBL and mediate uptake of the MBL-opsonized organisms. Ficolins are plasma proteins that are structurally similar to collectins. They possess a collagen-like domain, but instead of a C-type lectin domain, they have a fibrinogen-type carbohydrate recognition domain. Ficolins have been shown to bind several species of bacteria, opsonizing them and activating complement in a manner similar to that of MBL. MBL and ficolins associate with MBL-associated serine proteases (MASPs) including MASP1, MASP2, and MASP3. The MASPs are structurally homologous to the C1r and C1s proteases and serve a similar function, namely the cleavage of C4 and C2 to activate the complement pathway. ✓ The generation of C3 and C5 convertases by the three major pathways of complement activation. ✓ The classical pathway is initiated when C1q binds to antigen-antibody complexes. ✓ The antigen is shown here in dark red and the initiating antibody in green. ✓ The C1r enzymatic component of C1 (shown in blue) is then activated and cleaves C1s, which in turn cleaves C4 to C4a (an anaphylatoxin, bright red) and C4b. ✓ C4b attaches to the membrane, and binds C2, which is then cleaved by C1s to form C2a and C2b. ✓ (C2b is then acted upon further to become an inflammatory mediator.) ✓ C2a remains attached to C4b, forming the classical pathway C3 convertase (C4b2a). ✓ In the lectin pathway, mannosebinding lectin (MBL, green) binds specifically to conserved carbohydrate arrays on pathogens, activating the MBL-associated serine proteases (MASPs, blue). ✓ The MASPs cleave C2 and C4 generating the C3 convertase as in the classical pathway. ✓ In the alternative pathway, C3 undergoes spontaneous hydrolysis to C3(H2O), which binds serum factor B. ✓ On binding to C3(H2O), B is cleaved by serum factor D, and the resultant C3(H2O)Bb complex forms a fluid phase C3 convertase. Some C3b, released after C3 cleavage by this complex, binds to microbial surfaces. ✓ There, it binds factor B, which is cleaved by factor D, forming the cell-bound alternative pathway C3 convertase, C3bBb. ✓ This complex is stabilized by properdin. The C5 convertases are formed by the addition of a C3b fragment to each of the C3 convertases. The alternative pathway: ✓which was discovered later but is phylogenetically older than the classical pathway, is triggered when a complement protein called C3 directly recognizes certain microbial surface structures, such as bacterial LPS. ✓C3 is also constitutively activated at a low level in blood and extravascular fluid and binds to cell surfaces, but it is then inhibited by regulatory molecules present on mammalian cells. ✓Because microbes lack these regulatory proteins, the spontaneous activation can be amplified on microbial surfaces. ✓Thus, this pathway can distinguish normal self from foreign microbes on the basis of the presence or absence of the regulatory proteins. ✓ The lectin pathway is triggered by a plasma protein called mannose-binding lectin (MBL), which recognizes terminal mannose residues on microbial glycoproteins and glycolipids, similar to the mannose receptor on phagocytes described earlier. ✓MBL is a member of the collectin family (calcium dependent ) with a hexameric structure similar to that of the C1q component of the complement system. ✓After MBL binds to microbes, two zymogens called MASP1 (mannose-associated serine protease 1 or mannan-binding lectin-associated serine protease) and MASP2, with functions similar to those of C1r and C1s, associate with MBL and initiate downstream proteolytic steps identical to the classical pathway. Collectins A family of proteins, including mannose-binding lectin, that is characterized by a collagen-like domain and a lectin (i.e., carbohydrate binding) domain. Collectins play a role in the innate immune system by acting as microbial pattern recognition receptors, and they may activate the complement system by binding to C1q. C1, mannose-binding lectin, and ficolin. ✓These three homologous hexameric proteins can all initiate complement activation on binding to their ligands on cell surfaces. ✓C-type lectin–like globular heads (H) at the end of collagenous-like stalks in the C1q and mannose-binding lectin proteins bind the Fc regions of immunoglobulin M (IgM) or mannose on the surface of microbes, respectively. ✓Fibrinogen-like globular heads on ficolin bind N-acetylglucosamine on the surface of microbes. ✓ Binding results in conformational changes that activate the serine protease activity of C1r and C1s, associated with C1q, or mannose associated serine protease 1 (MASP1) and MASP2, associated with mannose binding lectin and ficolin.