I. Definition of antigensAntigens are the substance which when introduced parenterally intothe body stimulates the production of an antibody with which itreacts specifically and in an observable manner. Specificity is referred to that, immune responses are directed toward and able todistinguish between distinct antigen or small parts of macromolecular antigens.Ab1 Ab2 Ab3
How Antigen entersSites of antigen entrySites ofinitial antigen captureSites of antigencollection and capture
1.Immunogen:the antigenthat inducespecificimmuneresponse2.Tolerogen:antigen thatinduceImmunologictoleranceImmunologictolerance isunresponsivenessto an antigen thatis induced by priorexposure to thatantigen.3. Allergen: antigenthat induceAnaphylaxis (severeimmediatehypersensitivityreaction occurring asa result of rapidgeneralized mast-cellgranulation)4. Vaccine:antigens thatinduce aprotectionimmuneresponseagainstmicrobes andare used topreventdiseases
Immunogenecity vs AntigenicityImmunogenicity is the ability to induce a humoral and/or cell-mediatedimmune response.B cells + antigen effector B cells + memory B cellsT cells + antigen effector T cells + memory T cellsAntigenicity is the ability to combine specifically with the final products ofthe immune response (i.e. secreted antibodies and/or surface receptors on T-cells).Although all molecules that have the property of immunogenicity also have theproperty of antigenicity, the reverse is not true.
I) Nature of Immunogen: Foreigness: In order to elicit an immune response a molecule must berecognized as nonself by the host.What kinds of substances can be foreignness to immune system?(1) Heterogeneous substancesVarious pathogens, xenoantigeneic tissues.(2) Allogeneic substancegrafted allogeneic tissues or organs.(3)Autoantigenic components that never contact with lymphocytes duringembryo period. Molecular Size: Usually the bigger the better. Molecules with MW of5000-10000 are poor immunogens with the best immunogens being about100,000 D.
Chemical complexity : Just because a molecule is large, if its a polymer of asingle amino acid or sugar it tends to lack immunogenicity. The addition of aromaticamino acids such as tyrosine and phenylalanine has a profound effect on theimmunogenicity of these synthetic polymers. All 4 levels of protein organization ,primary, secondary, tertiary, and quaternary- contribute to the structural complexityof a protein and hence affect its immunogenicity.Degradability: Macromolecules that cannot be degraded and processed byAntigen presenting cells are poor immunogensII) Contribution of the Biological System: which includes1.Genotype of the recipient animal: The genes that code for MHCmolecules, T cell receptors, and B cell receptor all play a central role indetermining the degree of immune responsiveness to an antigen.2. Age : can also influence immunogenicity. Usually the very young andthe very old have a diminished ability to mount and immune response inresponse to an immunogen.
III) Method of Administration Dosage: Too low a dose of Antigen will fail to activate enough lymphocytesfor a response whereas too high a dose can overwhelm the system and cause thelymphocytes to enter a nonresponsive state.Route : Generally the subcutaneous route is better than the intravenous orintragastric routes. The route of antigen administration can also alter the natureof the responseAdjuvants : Adjuvants are substances that when mixed with an Antigenserve to enhance the immunogenicity of that Antigen. Adjuvants are oftenwater in oil mixtures with various bacterial components added. Aluminumpotassium sulfate (alum) is the only approved adjuvant for human use.
Classification of AntigensComplete antigenWhich contains bothimmunogenicity andantigenicityIncomplete antigenContains only antigenicitye.g. HaptensProteinsMajority of immunogens are proteins(pure proteins or they may beglycoproteins or lipoproteins). Proteinsare usually very good immunogens.Polysaccharides Pure polysaccharides andlipopolysaccharides are goodimmunogens.Nucleic AcidsNucleic acids are usually poorlyimmunogenic. However, they maybecome immunogenic when singlestranded or when complexed withproteins.LipidsIn general lipids are non-immunogenic,although they may be haptens.TD-Ag (thymus dependent antigens ) TI-Ag (thymus independent antigens)
1. Antigen determinants (epitope)The portion of antigen molecules which can be specificallyrecognized by antibody or antigenic receptor of lymphocytes.
Classification of antigenic determinant1.According to the structure of Antigen determinants Conformational determinants : areformed by amino acidresidues that aren’t in asequence but becomespatially juxtaposed inthe folded protein Sequential (or linear) determinantsEpitopes formedby several adjacentamino acid residuesare called lineardeterminants.
2.According to types of cells recognizing antigenic determinantsT cell epitope B cell epitopeReceptor TCR BCRNature short peptide proteins, polysaccharidesSize 8-17 amino acid residues 5-15 amino acid residuesor 5-7 monosaccharidesTypes linear epitope conformational epitopeor linear epitopePosition any position in antigen mostly exist on the surface ofantigen
References1.Immunology by Kubby2.www.google.com- antigens.pdf