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Antigen and antibody by Dr. Himanshu Khatri
- 1. Antigen and antibody Dr.Himanshu Khatri Email: [email protected]
- 2. Definition • An antigenis any substance which, when introduced into the body, usually stimulates the production of an antibody • It reacts with it specifically, and in an observable manner
- 4. Attributes of antigenicity •Immunogenicity: induction of immune response • Immunological reactivity: specific reaction with antibodies or sensitised cells
- 5. Antigen, hapten andadjuvant • Antigen exhibits both immunogenicity and immunological reactivity • Hapten exhibits only immunological reactivity • Haptens can exhibit immunogenicity on combining with larger carrier molecule • Adjuvants are substance which, when mixed with antigens, increase their immunogenicity
- 8. Antigenic determinant orepitope • Smallest unit of antigenicity • Small area of antigen consisting of 4-5 amino acids or monosaccharide residues • Having a specific chemical structure, electrical charge and spatial configuration • Capable of immunogenicity
- 11. Cross reactivity • Stereochemicalsimilarities is responsible for most cases of cross-reactivity • The presence of same or similar epitopes on different antigens is responsible for some of the antigenic cross reactions
- 12. Paratope • The areaon the antibody molecule corresponding to the epitope
- 13. Determinants of antigenicity •Size • Chemical nature • Susceptibility to tissue antigens • Foreignness
- 14. Size • Antigens withlarger molecular size are more immunogenic • Smaller molecules are less immunogenic, and may be related to hypersensitivity
- 15. Chemical nature • Proteinsand polysaccharides are more immunogenic • Lipids and nucleic acids are less immunogenic • Proteins are more immunogenic than polysaccharides
- 16. Susceptibility to tissueenzymes • Only substances which can be degraded by enzymes to immunogenic fragments are capable of being antigens • Non- degradable substances like polystyrene latex are non-immunogenic
- 17. Foreignness • Only antigenswhich do not belong to the self are immunogenic • This is because the body develops tolerance to self antigens during development of immune system • Antigens from other individuals of the same species are less than those from other species • Antigens from related species are less immunogenic than those from distant species
- 18. Antigen specificity • Speciesspecificity • Isospecificity • Autospecificity • Organ specificity • Heterogenetic (heterophile) specificity
- 19. Species specificity • Tissuesof all individual in a species contain species specific antigen
- 20. Isospecificity • Some antigensare found in some, but not all, members of a species • For example, the blood group antigens in humans are of two types A and B. People having antigens A, B, both, and none on RBCs are designated blood groups A, B, AB, and O respectively
- 21. Autospecificity • Some antigensare specific to an individual • The immune system does not respond to that antigen, because during development of immune apparatus, the immune system develops tolerance to that antigen
- 22. Organ specificity • Someantigens are specific for organs, and may be present in different species • The brain of sheep and humans share a common antigen. This is why, when people are vaccinated against rabies with vaccine prepared from sheep brain, the immune response attacks the human brain, and causes neuroparalytic complications
- 23. Heterophile specificity • Sameor similar antigens may be shared across species, classes, and kingdoms • For example, Rickettsia and Proteus share some same antigens. So, the antigens from Proteus species can be used to detect antibody response against Rickettsia species. This is the basis of Weil-Felix test
- 24. Biological classes ofantigens • Antibodies are secreted by plasma cells. Plasma cells are differentiated B cells • For the full expression of function, some cooperation from T-cells is necessary, and the antigens are first recognized by the T-cells (T- dependant or TD antigens) • However, some antigens can be directly recognized by B-cells, bypassing the recognition by T-cells (T-independent or TI antigens)
- 25. • TI antigens 1.Usually polysaccharide or lipopolysaccharide (LPS) 2. Too little amount is non- immunogenic and too much results in tolerance 3. Immunological memory is absent • TD antigens 1. Usually proteins 2. Immune response is not dose dependant 3. Immunological memory is present
- 26. Antibodies • Antibodies areproteins in the serum which react with an antigen specifically, and in an observable manner • They are a part of a globulin fraction of the serum
- 27. Antibodies - Immunoglobulins •Immunoglobulins are proteins of animal origin which have a known antibody activity or chemically related proteins without an antibody activity • All immunoglobulins do not have antibody activity, but all antibodies are structurally immunoglobulins • Immunoglobulins is a structural concept, whereas antibody is a functional concept
- 28. Structure of immunoglobulins •Each immunoglobulin (Ig) molecule consists of two heavy (H) and two light (L) chains • Each L chain is linked to each H chain with the help of a disulphide bond • Two H chains are linked to each other with the help of a disulphide bond
- 29. H chains • Thereare five classes of Ig molecules depending on the type of H chain in them Immunoglobulin class H chain IgG γ ( gamma) IgA α (alpha) IgM μ (mu) IgD δ (delta) IgE ε (epsilon)
- 30. L chains • Lchains occur in two varieties, kappa (κ) and lambda (λ) • A molecule of Ig may have two kappa or two lambda chains, but never one of each
- 32. Constant and Variableregions • The H and L chains of each Ig molecule consists of constant regions (which are structurally same in all Igs belonging to a class (γ, α, μ, δ, or ε) and variety (κ or λ) • The H and L chains also contain variable regions, which differ in their structure across IgGs • The variable region is found towards the antigen binding terminus of the IgG
- 33. Hypervariable regions • Thevariable regions contain a relative invariable region and some highly variable regions (or hot spots) • The sites on the highly variable regions, which make contact with the epitope, are called complementarity determining regions or CDRs
- 34. Fc and Fabfractions • The enzyme papain acts at the hinge region of the Ig molecule, and divides it into one Fc and two Fab fragments • The Fab fragment (ab for antigen binding) contains the antigen binding site • The Fc fragment (c for crystallisable) contains the site for binding of the complement and macrophages
- 35.
- 36. IgG • Most abundantof all Igs (80%) • Monomer • Divided equally in intra- and extra-vascular space • Only Ig to cross placenta from the mother to fetus • Participates in most immunological reactions like complement fixation, precipitation, and neutralization of toxins and viruses • Has a very long half-life, and persists for long periods in the body
- 37. IgA • Occurs asserum IgA and secretory IgA • Secretory IgA (SIgA) is a dimer, where the two IgA molecules are joined by a J chain (J for joining) • SIgA is found at mucosal surfaces and in secretions • SIgA also contains a secretory piece, which protects it from degradation by enzymes • It does not fix the complement, but can activate the alternate complement pathway
- 38. IgM • Pentamer • Fivemolecules of Ig are joined by a J chain • Mostly (80% of IgM) intravascular is distribution, IgM deficiency is associated with septicemia • Earliest Ig to be synthesized during immune response • Very short half life, hence their demonstration indicates recent infection • Does not cross the placenta, and hence used for detection of infection in neonates • Very strong agglutination and lysis action • Monomeric IgM is the major Ig on surface of B lymphocytes for antigen recognition
- 39. IgD • Mostly intravascular •Present on the surface of B lymphocytes, along with monomeric IgM, for antigen recognition
- 40. IgE • Mostly extravascularin origin • Present on the surface of mast cells and basophils • Levels increase in atopic conditions like asthma, hay fever, etc., • Levels also increase during parasitic infections
- 41. Functions of Igs •Igs are produced by the plasma cells (differentiated B cells), and perform the following functions: 1. They prevent attachment of microorganisms to mucosal surfaces (IgA) 2. They neutralize toxins of microorganisms 3. They facilitate binding of complement, and activation of its pathway 4. They opsonize the microorganisms for more effective phagocytosis
- 42. Abnormal immunoglobulins • Theyare seen in: 1. Multiple myeloma 2. Waldenstrom’s macroglobulinemia 3. Cryoglobulinemia
- 43. Multiple myeloma • Cancerousplasma cells produce large amounts of Igs • Plasma cells producing IgG, IgA, IgD or IgE may be affected
- 44. Waldenstrom’s macroglobulinemia • Cancerousplasma cells produce large amounts of IgM
- 45. Bence Jones proteins •Are light chains • Are either κ or λ in any single patient • Can be identified in urine by its typical property of coagulation at 50oC, but redissolving at 70oC • Are usually found in Multiple Myeloma, but may be also found in Waldenstrom’s macroglobulinemia
- 46. Cryoglobulinemia • Cryoglobulins areimmunoglobulins or their fragments; or proteins of the complement system • They precipitate on cooling but redissolve on warming • They are not only found in multiple myeloma and Waldenstrom’s macroglobulinemia, but also in autoimmune conditions like systemic lupus erythematosus
- 47. Self study • Immunoglobulinspecificities • Class switching in immunoglobulins • Antibody diversity
- 48.