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Chapter: Biochemistry: Immunology


An antigen is a foreign substance, which is recognized by the immune system.



An antigen is a foreign substance, which is recognized by the immune system. Antigens can be defined as a substance that can combine specifically to the components of immune response such as lymphocytes and antibodies. An immunogen is any substance that has the ability to evoke B or T or both B and T mediated immune reactions. Whole antigen cannot combine with the antibody as antibodies are formed against specific regions on the surface of an antigen called antigenic determinantor epitopes.


1. Structure and Types of Antigens


An antigen molecule may contain a number of similar group or different antigenic determinant. The figure 10.4 shows that a cell which contains different groups of molecules over the surface. However only the group a andd has been selected for antigen processing. Hence a andd are antigenic determinant. Normally antigens are multi determinant.

Types of antigen


Antigen possesses several unique molecular structures which can induce an immune response. Most antigens are proteins, nucleoproteins, lipoproteins,

glycoproteins, or large polysaccharides with a molecular weight greater than 10,000. To become an antigen the molecule must be relatively having a higher molecular weight. Large antigenic molecule posses many antigenic determinant per molecule. However the low-molecular-weight substance that can combine with an antibody but cannot induce the formation of antibodies are called as haptens. They can also initiate antibody response when they are combined covalently with a carrier molecule. Since antigens stimulate the immune response they are other wise called as immunogens.


2. Factors influencing the antigenicity of antigens


Antigen must be a foreign substance as more foreign the substance, the more immunogenic in nature. However the following factors can also influence it,


1.           The antigenic response which is indicated by the quantum of antibody formed in response to antigenic stimulation varies depending on the dosage of antigenadministered, route of administration and use of adjuvant etc.


2.           Molecular weight of the antigen affect the antigenicity as low molecular antigenscan only combine with the antibody (hapten).


3.           Very low molecular weight substance cannot act as an antigen. Because of this the virus which has the very low molecular weight proteins escapes the immune response.


4.           Very large molecular antigen directly induces the B cell differentiation with out the involvement of T Cells.


5.           Degradability is essential as in the antigen presenting cells process the antigenby degrading them and processed peptide antigen along with the MHC II molecule presented to the T cells and such antigens are called T dependent antigen.


6.           Antigen induced antibody response can be suppressed by administrating the antibody passively either prior to or shortly after administration of antigen (This is utilized for the treatment of Rh antigen induced antibody in the mother leading to Erythroblastosis fetalis).


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