Immunogen & Antigen. Dr Ola Ibrahim Ahmed Professor of Microbiology& Immunology Faculty of Medicine Ain Shams University

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1 Immunogen & Antigen Dr Ola Ibrahim Ahmed Professor of Microbiology& Immunology Faculty of Medicine Ain Shams University

2 By the end of this lesson the student is expected to Define antigen and immunogen. Define hapten. List factors that influence immunogenicity. Define heterophil antigen and give examples.

3 Antigen Any agent capable of binding specifically to component of immune response (antibodies and activated T cell) when introduced into the body. Many substances are capable of binding with components of immune response though they are incapable of inducing an immune response

4 Immunogen Any agent capable of inducing an immune response All immunogens are antigens, but not all antigens are immunogens Chemicals with low molecular weight as penicillin bind to antibodies and therefore they are antigens but can't activate B cells on their own (not immunogenic).

5 To generate antibodies specific for such chemicals immunologist attach them to proteins. In this case the low molecular weight substance is called hapten and the protein to which it is conjugated is called a carrier. The hapten carrier complex can act as immunogen

6 Haptens Are low molecular weight compounds Can`t induce immune response But when conjugated to carrier molecules, an immune response develops against both hapten and carrier.

7 The antibiotic penicillin is an example of hapten with low molecular weight that does not induce immune response But sometimes it binds to serum proteins inducing immune response against both penicillin and serum proteins

8 Factors that influence immunogenicity 1. Foreignness 2. Chemical Composition Proteins are more immunogenic than polysaccharides. Lipids and nucleic acids do not serve as immunogens unless they are complexed with a protein molecule.

9 3. Molecular Size 4. Dosage 5. The route of administration Antigens injected Subcutaneously, Intramuscular or Intradermal generally elicit the strongest responses due to slow rate of absorption

10 An epitope (antigenic determinant) Immune cells do not interact with or recognize an entire foreign molecule; instead lymphocytes recognize discrete sites on the macromolecule called epitopes or antigenic determinants

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12 An epitope serves as the smallest biochemical unit of an antigen that is capable of eliciting an immune response One antigen may have several identical or different epitopes, for example, a carbohydrate with repeating sugar units has several identical epitopes, while a protein molecule has many different antigenic epitopes

13 Cross Reactivity: Antigens sharing one or more epitopes are called heterophil antigens. Heterophil antigens can stimulate the production of heterophil antibodies that react with shared epitopes of heterophil antigens.

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15 Practical Applications: a- Immunopathogenesis of rheumatic fever: Heterophil antibodies produced against group A hemolytic streptococcus M protein cross react with the mammalian heart protein myosin.

16 b. Detection of heterophil antibodies in patient s serum is a helpful laboratory test for diagnosis of many diseases as Paul-Bunnel test in diagnosis of infectious mononucleosis: Heterophil antibodies present in the serum of patients infected with Epstein- Barr virus cross react with sheep RBCs.

17 Weil Felix test Patients with rickettsial infections as typhus fever have in their serum heterophil antibodies that can cross react with some strains of Proteus vulgaris OX19, OX2, and OXκ and agglutinate them.

18 Tests used for diagnosis of syphilis Patients with syphilis (caused by Treponema pallidum) have in their serum heterophil antibodies that can cross react with cardiolipin Ag (consists of beef heart muscle extract + cholesterol + lecithin).

19 Heterophil antigens - Heart muscle (rheum fever) - Proteus (Weil-Felix test) - Sheep RBCs (Paul bunnel test) - Cardiolipin Ag 2 Ag 1 -Strept M ptn - Rickettsia - EBV -T.pallidum Heterophil Ab

20 Pop quiz Define 1. Immunogen 2. Antigen

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