A antigen is a Macromolécule natural or synthetic, foreign at the organization considered, but recognized by Anticorps or cell S of the Immune system and able to generate an immunizing answer .
Les antigens is generally Protéine S, Polysaccharide lipidic S and their derivatives . Antigen fragments called Haptène S can also induce a Allergie.
The antigens, as markers of the foreign agents at the organization, are at the base of the immunizing response. It is the recognition of the antigen by the cells immunocompétentes, directly or via the cells presenters of antigen (CPA), which activates the immune system.
In the case of proteinic antigens, one names épitope or antigenic determinant the part of the antigen recognized by a lymphocytary antibody or receiver. The same antigen can comprise several épitopes (identical or different) and thus induce a varied immunizing response. There exist sequential épitopes , corresponding to a sequence of amino-acid, and épitopes conformational , related to the structure of protein and thus sensitive to the denaturation. The recognition of the antigen by the Lymphocyte S depends on the nature of the épitope. The lymphocytes B bind directly to the conformational épitopes thanks to the Immunoglobuline S of their membrane. The lymphocytes T recognize the sequential épitopes presented by the cells presenters of antigen.
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