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Antigens

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Published in: Biology
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Any substance that elicits antibody mediated immune response. The following ppt file elucidates the different types of antigens and their characteristics.

Soham J / Pune

1 year of teaching experience

Qualification: Int. M.Sc. Biotechnology

Teaches: Biology, Chemistry, English, EVS, Algebra, Zoology, Bio Chemistry, Bio Technology, Bio-informatics

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  1. ANTIGENS
  2. What is an Antigen? Any substance that can be recognized by the Ig of B cells or by the TCR when complexed with the MHC is an Antigen. Immunogenecity Ability to induce humoral or cell- mediated immune response. All immunogens are antigens Antigenecity Ability to combine specifically with the products of humoral or cell- mediated immune response. All antigens are not immunogens. Haptens: Antigenic but not immunogenic
  3. What factors influence Immunogenecity? Nature of the Immunogen ' Non-self , phylogenetic distance , sequestered antigens Foreignness ' > 100kDa or more are active immunogens. Molecular Size Chemical composition & Heterogeneity With exceptions: dextran Low mow. chemicals : formaldehyde Synthetic homopolymers lack immunogenecity. Copolymers composed of different amino acids / sugars immunogenic than homopolymers. Complexicity imp factor Structural stability important. Ex: gelatin , proteins with aromatic amino acids Large insoluble macromolecules> immunogenic soluble ones Susceptibility to Macromolecules not degraded — poor immunogens Ag processing & D-amino acids - poor immunogens Presentation L-amino acids - good immunogens. Lipid-proteins as hapten-carrier conjugates. Recognition of lipids by T cells through CDI Lipids
  4. Response of the Host (biological system) receiving the Antigen is important Genotype of recipient and Age Different genotype respond differently to immunogens. ----different Ab response. ex: same Ag The kind of immune responsiveness depends upon MHC genes of the host. ' Impaired at very young and old age. Adjuvants Adjuvants enhance immunogenecity of the Ag. Augment immune response by : Dosage & route of administration Each immunogen has a dose-response curve. High or low dose can induce tolerance. Ex. Pnemococcal capsular polysaccharide. Booster immunizations. iv,id , sc , im , ip 1. prolong Ag persistence 2. enhance co-stimulatory signals 3. increase local inflammation 4. stimulate non-specific proliferation of lymphocytes.
  5. N/A
  6. TABLE 3.2 POSTULATED MODE Of ACTION Of SOME COMMONLY USED ADJUVANTS Postulated mode of action Prolongs antigen persistence Enhances costimulatory signal Induces granulorna format ion Stimulates lymphocytes nonspecifically Adjuvant Freund's incomplete adjuvant Freund's complete adjuvant Aluminum potassium sulfate {alum Mycobacterium tuberculosis Bordetejlo pertussi$ Bacterial lipopolysaccharide CLPS) Synthetic polynucleotides (poly IC/poly AL.T}
  7. TYPES OF ANTIGENS Heterophile antigens Antigens that are shared b/w related species. ' Forsmann antigen. Shared antigens b/w Proteus and Rickettsia Weil-Felix test. Isophile antigens Antigens carried by an individual of a species, capable of eliciting immune responses in genetically distinct member of same species. Blood group antigens. Sequestered antigens Sperm antigen & corneal tissue / lens antigen. Exposure during disease increases immunogenecity. Superantigens Bacterial toxins Bind directly to MHC molecule at a distinct site without Ag-processing. massive release of cytokines, immunosuppressive symptoms: nausea, vomiting , hypotension. Fatal at times.
  8. EPITOPE Discrete sites on an immunogenic macromolecule. ' Immunologically active regions of an immunogen. T cells and B cells recognize different epitopes on the same antigenic molecule. Lymphocytes interact with a complex antigen on several levels of antigen structure. TABLE 3-3 COMPARISON OF ANTIGEN RECOGNITION BY T CELLS AND B CELLS Characteristic Interaction with antigen Binding of soluble antigen Involvement Of MHC molecules Chemical nature Of antigens Epitope properties B cells Involves binary complex of membrane Ig and Ag Yes None required Protein, polysaccharide, lipid Accessible hydrophilic, mobile peptides containing sequential or nonsequential amino acids T cells Involves ternary complex oft-cell receptor, AB, and MHC molecule No Required to display processed antigen Mostly proteins, but lipids and glycolipids presented on MHC-like molecules Internal linear peptides produced by processing of antigen and bound to LMI IC molecules
  9. Properties of B cell epitopes The ability to function as a B cell epitope is determined by the nature of the antigen- binding site of the antibody molecules displayed by B cells. ' Complementary shapes of Ag-binding site and the epitope Strong bond Size of the epitope not larger than the size of the Ag-binding site of the Ab. Shape of the epitope Shape assumed by the sequence of amino acids in the binding site. ' Larger areas of the globular protein Ag are engaged by the binding site. Shape of epitope : tertiary conformation of native protein. ' In contrast, small peptides fold into compact structures that occupy less space and fit into cleft of the binding site.
  10. Interaction of globular proteins with Antibody molecule
  11. Properties of B cell epitopes The B cell epitopes on native proteins generally are composed of hydrophilic amino acids on the protein surface that are topographically accessible to the membrane- bound or free antibody. Must be accessible. ' Protruding regions most likely to be recognized as epitopes — predominantly hydrophilic amino acids. ' Amino acid sequences in the interior of the protein- hydrophobic- not function as B cell epitopes. ' Complementary protrusions and depressions. Bonds: Hydrogen bonds, ionic and hydrophobic interactions. Ant' en Antibod
  12. Properties of B cell epitopes B cell epitopes can contain sequential or nonsequential amino acids. ' Sequential contiguous residues. Or non sequential residues from the segments of the chain brought together by the folded conformation of an (145) 146 151 COOH NH2 antigen. Antibodies to native protein do not bind to the denatured protein. Binding of Ab to Ag depends on maintenance of the tertiary structure of the epitopes by intrachain disulphide bonds. Heme 56-62 15-21 (22) 113-119 Disulfide bond (a) Hen egg-white lysosome H2N 64 COOH 80
  13. Properties of B cell epitopes B cell epitopes tend to be located in the flexible regions of an immunogen and display site mobility. Major Antigenic determinants in proteins generally located in the most mobile regions. Site mobility of epitopes maximizes complementarity with the Ag binding site. ' But is of lower affinity due to loss of entropy. Complex proteins contain multiple overlapping B cell epitopes, some of which are immunodominant Most of the surface of a protein is potentially antigenic. ' Subset of antigenic sites on a given protein recognized by the immune system is much smaller than the potential antigenic repertoire. ' Immunodominant epitopes induce a more pronounced immune response than other epitopes of the same protein.
  14. Properties of T cell epitopes Antigen-derived peptides ANTIGEN RECOGNITION BY T AND LYMPHOCYTES TABLE 3-4 REVEALS QUALITATIVE DIFFERENCES Secondary immune response Primary immunization Native protein Native protein Secondary immunization Native protein Denatured protein Antibody production Cell-mediated T response* is of cells nwdi*te cell-mediated nsponse called delayed-type hypersensitivity 14}. Antigen-presenting cell Presentation of peptides in context of MHC molecule MHC Formation of CD4 Antigenic peptide complex TCR Class 11 MllC Agretope Epitope TCR TH cell
  15. Antigen processing is required to generate peptides that interact specifically with MHC molecules Endogenous antigens processed into peptides within the cytoplasm Exogenous antigen processed by the endocytic pathway. Binding of MHC to Antigenic peptide does not have the fine specificity of the epitope-Ab interaction. A MHC molecule can bind to a wide variety of peptides. Antibody can bind to only that epitope for which it is specific. Epitopes recognized by T cells are often internal T cells recognize internal peptides that are exposed by processing within Ag-presenting cells or altered self-cells.
  16. HAPTENS Antigenic but not immunogenic Haptens — small molecules Large carrier protein Hapten-carrier conjugate (immunogenic) Carrier Haptcn Immunize rabbit Haptcn-carricr conjugate Injection with: Hapten (DNP) Protein carrier (BSA) Hapten-carrier conjugate (DNP-BSA) Antibodics to haptcn Antibodies to carrier Antibodies to conjugate of hapten and carrier Antibodies formed' Nonc Anti-BSA Anti-DNP (major) Anti-BSA (minor) Anti-DNP/BSA (minor)
  17. REACTIVITY OF ANTISERA WITH VARIOUS HAPTENS Reactivity with Antiserum against Arninobenzene a-Arninobenzoic acid rn-Arninobenzoic acid p-Amjinobenzoic acid Antiserum against Aminobenzene p-Chloroaminobenzene p-Toluidine p-Nitroan•inobenzene O Aminobenzene (aniline) Aminobenzene (aniline) O 0—Arninobenzeic acid 0 NM-Arai rtc•bettzoie acid Reactivity with p—ChLoroamino benzene p-AminoE•enzoie acid O O .p—Nitreamino benzene KEY.' O no reactivity-v + + and -E +- + -E strong reactivity; -E -f- and + Lesser degrees of reactivity SOL.TRCE.; K Specificity J Seie•re John Wiley
  18. Pattern Recognition Receptors Receptors of innate immunity Recognize broad structural motifs that are highly conserved within microbial species but absent from the host. Recognize combination of sugars, proteins, lipid-bearing molecules , nucleic acid motifs. Do not recognize self antigens. Proteins by nature. Genes encoding PRRs present in the germline of the organism. CRP , MBL , Scavenger receptors , TLRs
  19. Characteristic Specificity Self/nonself discrimination Receptor {location} Antibody (B-cell membrane, bloodi tissue fluids) T-cell receptor IT-cell membrane) Innate immunity Specific for conserved molecular patterns or types Perfect: evolutionarily selected to distinguish phylogenetic differences, Never recognizes self. Adaptive immunity Specific for details of antigen structure Excellent: but imperfect Occasional reaction with self antigens RECEPTORS OF THE ADAPTIVE IMMUNE SYSTEM Target {source) Specific components of pathogen Proteins or certain lipids of pathogen Ef&ct of recognition Labeling of pathogen for destruction and removal Induction of pathogen- specific hu moral and cel l- mediated Immunity
  20. RECEPTORS OF THE INNATE IMMUNE SYSTEM Complement (bloodstreamf tissue fluids) Mannose-binding lectin {MBL) (bloodstreamf tissue fluids) C-reactive protein (CRP) (bloodstreamt tissue fluids) LPS-binding protein (L8P) (bloodstreamt tissue fluids) TLR2 (cell membrane} TLR3 (cell membrane) TLR4 (cell membrane} TLR5 (cell membrane} TLR9 (cell membrane) Scavenger receptors (many) (cell membrane} Microbial cell-wall components Man nose-containing microbial carbohyd rates (cell walls) Phosphatidylcholine (microbial _membranes) Bacterial lipopolysaccharide (LPS} Cell-wall components of gram-positive bacteria, LPS*. Yeast cell-wall component (zym0san) Double-stranded RNA (dsRNA) (replication of many RNA viruses) LPS* Flagellin (flagella ofgram-positive and gram-negative bacteria) cpG Many targets; gram-positive and gram- negatWe bacteriat apoptotic host cells Complement activationt opsonization Complement activation, opsonization Complement activation, opsonization Delivery to cell-membrane LPS receptor (TLR-C014-MD-2 Attracts phagocytesf activates macrophages, dendritic cells. Induces secretion of several cytokines Induces production of interferon, an antiviral cytokine Attracts phagocytesf activates macrophagesi dendritic cells. Induces secretion of several cytokines Attracts phagocytesf activates macrophages, dendritic cells. Induces secretion of several cytokines Attracts phagocytesf macrophages, dendritic cells. Induces secretion of several cytokines Induces phagocytosis or endocytosis LPS is bound at the cell membrane by a complex of proteins that includes CO 14, MO-2, and a TLR

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