Antigens and Antibodies.pdf

Antigens and Antibodies to become immunogenic. All Immunogens are Antigens but not all Antigens are - Foreignness - Antigens must be recognized as Immunogens....

Antigens and Antibodies to become immunogenic. All Immunogens are Antigens but not all Antigens are - Foreignness - Antigens must be recognized as Immunogens. foreign to trigger an immune response. Antigens - Substances that can stimulate the production of Immunogens and Antigenicity antibodies and bind to them. - Foreign molecules that trigger a specific immune Immunogens and Antigens response; can include components of bacteria, - Immunogens trigger an immune response, while viruses, fungi, and protozoa antigens may or may not. Example: Bacterial cell wall components as Antibodies potent immunogens. - Proteins produced by B cells that specifically recognize and bind to antigens. - Antigenic Determinants (Epitopes) - Specific - Immunoglobulins (Ig) that bind specifically to parts of an antigen that are recognized by antigens at their antigen-binding sites; play a antibodies. crucial role in the humoral immune response - Multiple Epitopes - A single antigen can have several epitopes, each capable of binding Importance different antibodies - Crucial for immune defense against pathogens. - Foundation for vaccine development and disease prevention. - Integral to understanding autoimmune diseases and hypersensitivities. Example: The role of antigens in triggering an immune response during infections Antigens - Body does not direct immune response against whole bacteria, fungi, protozoa or viruses. - Foreign molecules trigger a specific immune Chemical Nature of Antigens response. - Include components of bacterial cell walls, Chemical Composition capsules, pili, and flagella, as well as proteins of - Antigens can be proteins, polysaccharides, viruses, fungi and protozoa. lipids, or nucleic acids. - Food and dust can also contain antigenic Proteins particles. - The most potent and complex antigens due to - Enter the body by various methods: their diversity and structural complexity - Through breaks in skin & mucous Haptens membranes - Small molecules that are antigenic but not - Direct injection, as with a bite or needle immunogenic unless attached to a carrier. - Through organ transplants and skin Example: Penicillin as a hapten that can cause grafts allergic reactions when bound to serum proteins. General Characteristics of Antigens - Immunogens - Macromolecules capable of inducing an immune response Hapten-Carrier Complex - Antigenicity - The ability of an antigen to bind to - Hapten alone cannot induce an immune specific antibodies or T cell receptors. response. - Not all antigens are immunogens: Small - When bound to a carrier protein, the complex molecules like haptens require a carrier protein becomes immunogenic. Clinical Relevance: Major Histocompatibility Complex - Understanding haptens is crucial for addressing drug allergies and designing conjugate vaccines. - “A complex of genes encoding cellsurface molecules that are required for antigen presentation to T-cells…” Physical Nature of Antigens - Fundamentally important: basis of self / not self distinction and presentation of processed antigen Molecular Weight - MHC-I (on nearly all nucleated cells) - Antigens typically have a high molecular weight, - MHC-II (on B-cells, macrophages, dendritic cells) which enhances immunogenicity. Structural Stability - Stable antigens are more likely to elicit a MHC Regions and Gene Encoding sustained immune response. - MHC Regions - Divided into four major regions: D, B, C, A. Class I Genes - Includes HLA-A, HLA-B, and HLA-C - which are involved in presenting peptides to Foreignness cytotoxic T cells. - Antigens must be sufficiently different from host Class II Genes molecules to be recognized as foreign. - Encoded in the HLA-D region, subdivided into Degradability HLA-DR, HLADQ, and HLA-DP - Antigens must be degradable by - responsible for antigen presentation to helper T antigen-presenting cells (APCs) for proper cells. processing and presentation. Antigen Coding - Each region codes for specific antigens, numbered for identification (e.g., HLA-A1, HLA-B2). Classes of HLA Molecules and Their Functions Class I HLA Molecules - Function in regulating interactions between cytolytic T cells and target cells. - Inhibited by CD8+ T cells. Class II HLA Molecules General Classes of Antigens - Regulate interactions between helper T cells and antigen-presenting cells (APCs). Exogenous - Inhibited by CD4+ T cells. - presented by Antigen Presenting Cells (APC’s). These are macrophages, B-cells, and some dendritic cells - these antigens are presented in MHC-II; they are seen by T-cells with a TCR and an associated protein called CD4 Endogenous - typically peptides derived from any protein; an infected cell displays “not-self” proteins and is, thus, an “altered self cell” - these antigens are presented by MHC-I; they are seen by T-cells with a TCR and an associated protein called CD8  What are the two types of T-cells Immunoglobulin Classes - TH have CD4 which interacts with MHC-II; thus, CD4+ T-cells are “MHC-II restricted.” Five Main Classes - TH cells are “helper cells” that send signals (via - IgG, IgM, IgA, IgE, and IgD. cytokines and surface proteins) to other cells of Each class has a unique structure the immune system. - Determining its specific function in the immune - The TH cells function as the “brain” of the response. immune system - TC have CD8 which interacts with MHC-I; thus, Example - IgG as the most abundant CD8+ T-cells are “MHC-I restricted.” immunoglobulin in serum, involved in secondary - TC cells become cytotoxic T lymphocytes (CTL’s) immune responses which attack “altered self-cells (e. g., infected cells.) “Altered self-cells” are also called “target cells.” - They are the targets for the CTL’s cytotoxicity. Antibodies Antibodies (Immunoglobulins) - Y-shaped proteins that specifically recognize and bind to antigens. - Considered part of the humoral immune response since bodily fluids such as lymph and blood were once called humors. - Produced by B cells upon activation by antigens. IgG (Globulin) Functions - IgG is the principal immunoglobulin of the - Neutralization of pathogens, opsonization for secondary response. phagocytosis, and activation of the complement - IgG is the principal immunoglobulin of the adult system. (but not the neonate.) Primary Role - IgG constitutes 80% of the circulating - Antibodies serve as the effector molecules in immunoglobulin in the adult. humoral immunity. - The concentration of IgG in serum is high: 8-16 Specificity mg ml-1. - Each antibody is specific to a particular antigen - The architecture of IgG is simple: epitope. - two gamma chains and two kappa chains Diversity - two gamma chains and two delta chains - Generated through genetic recombination, allowing the immune system to recognize a vast array of antigens. IgM (Macroglobulin) - a pentamer (in secreted form) attached by disulfide bonds between the C4 and C3 domains of adjacent heavy chains; - in addition, there is a single J-protein. - IgM is the first immunoglobulin to be synthesized in a primary response.  - The concentration of IgM in serum is ~ 1.5 mg IgE ml-1. - A trace plasma protein, crucial in mediating - IgM is the first immunoglobulin to accumulate in allergic reactions, anaphylaxis, and immunity the serum of neonates. against parasites. - IgM has a high valency which contributes to - Binds strongly to receptors on mast cells and agglutination. Also, multimeric nature contributes basophils, leading to the release of histamines to effective complement activation. and heparin during immune response - mIgM is a monomer Summary Characteristics: - IgG: Major antibody in secondary responses, crosses the placenta. IgA - IgM: First antibody produced in response to an - Most often a dimer; sometimes a tetramer (or a infection, pentameric structure. trimer). - IgA: Found in mucosal areas, important for - Multimers united by J-protein. mucosal immunity. - Represents 15%-20% of total circulating - IgE: Involved in allergic reactions, binds to mast immunoglobulins. cells and basophils. - Predominant in Secretions: Found in tears, - IgD: Functions mainly as a receptor on B cells, saliva, colostrum, milk, and intestinal fluids. role in initiating B cell activation. - Synthesis and Function: Produced by plasma cells on body surfaces; forms secretory IgA when Functional Differences: bound to the secretory component, protecting - IgG: Efficient in opsonization and complement against gastrointestinal digestion and guarding activation. mucosal surfaces against pathogens - IgM: Potent agglutinating antibody due to its pentameric form. - IgA: Prevents colonization by pathogens in mucosal areas. - IgE: Triggers histamine release during allergic reactions. IgD - Found in very low concentrations in plasma, Antibody Structure accounting for less than 1% of the total immunoglobulin pool. - Y-shaped Molecule: Composed of two heavy - Highly susceptible to proteolysis and primarily chains and two light chains. exists as a cell membrane-bound Ig on B - Fab Region: Binds to the antigen, determines lymphocytes, associated with IgM. specificity. - Fc Region: Mediates effector functions like binding to Fc receptors on immune cells - Fab (Fragment antigen binding) - Contains the variable regions that determine antigen specificity. - Fc (Fragment crystallizable) - Interacts with cell surface receptors and complement proteins. - Hinge Region - Provides flexibility to the antibody, allowing it to bind to antigens at various angles  Antigen-Antibody Interaction Specificity - ability of a particular antibody to combine with a particular antigen - resides in the portion of Fab molecule (combining site) - the higher the fit with antigenic determinant, the stronger are the noncovalent bonds, and the higher is the affinity Immunoglobulin Variants Cross-reactivity - occurs when some of the determinants of an - Isotype: Refers to the class of the antigen are shared by similar antigenic immunoglobulin (e.g., IgG, IgA). determinants of an unrelated molecules, a - Allotype: Genetic variations within a given portion of the antibodies directed against one immunoglobulin class, specific to an individual or type of antigen will also react with the other type population. of antigen - Idiotype: Unique antigen binding sites formed by Antibody Affinity the variable regions of the antibody. - the initial force of attraction that exists between a single Fab site on an antibody molecule and a single epitope on the corresponding antigen Antibody Avidity - the functional combining/binding strength of an antibody with its antigen Immune Complexes - the noncovalent combination of antigen with its respective specific antibody Antibody Synthesis Primary Antibody Response 1. Lag Phase: No detectable antibodies. 2. Log Phase: Antibody titer increases Molecular Basis of Antigen-Antibody Reactions logarithmically. 3. Plateau Phase: Antibody titer stabilizes. Types of Bonding 4. Decline Phase: Antibodies are catabolized. - Hydrophobic Bonds - the major bonds formed Secondary (Anamnestic) Response between antigens and antibodies - Faster Response: Shorter lag phase, longer - exclusion of water during the interaction plateau, gradual decline. of hydrophobic/nonpolar side chains of - Antibody Types: Predominantly IgG; some IgM. both antigen and antibody, resulting in a - Higher Titer: Plateau levels typically 10-fold gain in energy and formation of an higher than primary response. energetically stable complex - Hydrogen Bonds - formation of hydrogen bridges between appropriate atoms - O-H-O , N-H-N , O-H-N - Van der Waals Forces - nonspecific attractive forces generated by the interaction between electron clouds and hydrophobic bonds - Extremely weak  - Electrostatic Forces - result from the attraction of - Identifying and quantifying hormones oppositely charged amino acids located on the - Typing tissue and blood side chains of two amino acid residues - Identifying infectious agents - Identifying clusters of differentiation for the Goodness of Fit classification of leukemias and lymphomas and - strongest bonding develops when follow-up therapy 1. antigens and antibodies are close to each other - Identifying tumor antigens and autoantibodies 2. The shapes of the antigenic determinants and - Delivering immunotherapy the antigen binding site conform to each other Detection of Antigen-Antibody Reactions (In-vitro Tests) - Agglutination - the process whereby particulate antigens aggregate to form larger complexes in the presence of a specific antibody - Precipitation reaction - combination of soluble antigen with soluble antibody to produce insoluble complexes that are visible - Hemolysis testing - reaction of antigen and antibody with a cellular indicator - Enzyme-Linked Immunosorbent Assay (ELISA) and other immunologic methods Monoclonal Antibodies - Monoclonal antibodies (MAbs) are highly purified antibodies derived from a single cell clone, exhibiting exceptional purity and specificity for a particular antigen. - In 1975, Köhler, Milstein, and Jerne pioneered the hybridoma technique, combining lymphocytes with myeloma cells to produce immortal, antibody producing cells. They received the Nobel Prize in 1984 for this breakthrough Discovery and Hybridoma Technique - Initial Steps: Immunization of donors with antigens (e.g., sheep erythrocytes) to produce specific antibodies. - Cell Fusion: Lymphocytes fused with myeloma cells using Sendai virus to create hybridomas. - Selection: Hybridomas that produce the desired antibodies are identified and cloned. Hybridoma Cells -These cells secrete the specific antibody and can be cloned to produce monoclonal antibodies. Uses of Monoclonal Antibodies