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Antigen – Types, Epitopes, Paratop, by Imran Hussain
- 1. Introduction and Properties ImranHussain Assistant Professor & Head Department of Medical Laboratory Techniques Integral University, Lucknow Antigen
- 2. Terminologies Antigen - Immunogen - Hapten- Pathogen - Any agent (molecule) that binds to components of the immune response – lymphocytes and their receptors -- antibodies and the TcR. Any agent capable of inducing an immune response. is antigenic but not immunogenic unless it is attached to a carrier molecule that supplies the immunogenicity. A pathogen is defined as an organism causing disease to its host, with the severity of the disease symptoms referred to as virulence.
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- 4. Antigen & AntibodyInteraction
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- 6. Types of Antigen •Endogenous Antigen • Exogenous Antigen • Auto Antigen • Tumor Antigen
- 7. Endogenous Antigen • Endogenousantigens are antigens that originate within the body of the host during metabolism or as a result of intracellular viral or bacterial infection. • Endogenous antigens are usually the cells of the body or fragments, compounds, or antigenic products of metabolism. • These are usually processed in the macrophages and are later detected by cytotoxic T-cells of the immune system.
- 8. Exogenous Antigen • Exogenousantigens are the antigens that originate outside the body of the host and, thus, are foreign to the host. • These antigens might enter the body through inhalation, ingestion, or injection and then circulate throughout the body via bodily fluids. • The uptake of exogenous antigens is primarily mediated by phagocytosis via Antigen Processing Cells (APCs) like macrophages, dendritic cells, etc.
- 9. Auto Antigen • Autoantigensare proteins or protein complexes of the host that are attacked by the host’s immune system, resulting in autoimmune disease. • Autoantigens can be deadly to the host as the body’s own cells should not be targeted by the immune system. • The immunological tolerance to such antigens is lost as a result of genetic and environmental factors.
- 10. Tumor Antigen • Tumorantigens or Neoantigens are presented by Major Histocompatibility Complex (MHC) I and II on the surface of tumor cells. • The antigens are produced as a result of a tumor-specific mutation during the malignant transformation of normal cells. • These antigens usually do not induce an immune response as the tumor cells develop ways to evade antigen presentation and immune defense.
- 11. Properties of Antigen •Foreignness • Molecular size • Heterogeneity • Chemical Nature
- 12. Foreignness • All antigensthat induce an immune response in the host are foreign to the body of the recipient. • The host body recognizes the antigen to be different from the normal body components. • The immunogenicity of the antigen increases with the increase in the degree of foreignness.
- 13. Molecular Size &Heterogeneity • The molecular size of the antigens is also crucial in the immunogenicity of the molecules. • It has been established that antigens should have a minimum size of greater than 5000 Da before they can be considered immunogenic. • However, low molecular weight substances can demonstrate immunogenicity when coupled with large-sized carriers. • The low molecular weight substances are termed haptens and are considered ‘partial antigens’ with at least one antigenic determinant.
- 14. Chemical Nature • Themost potent and commonly encountered antigens are proteins followed by polysaccharides. • However, other molecules like lipids and nucleic acids can also act as antigens when complex with proteins and polysaccharides.
- 15. B and T- Cell Epitopes T-cell Epitopes are usually protein antigen-derived peptides presented by MHC molecules on antigen-presenting cells and recognized by T-cell receptors. B Cell Epitopes are either peptides or protein surface residues that bind to an antibody.
- 16. T Dependent andT Independent Antigens The activation of B cells may or may not require the help of T cells. Antigens that require the help of T cells for B cell activation are thymus-dependent (T-dependent) antigens. Antigens that can stimulate B cells without the help of T cells are thymus- independent (T-independent) antigens.
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