An antigen is part of a (usually foreign) protein that is recognized by an antibody. Antibodies are proteins with highly variable regions that bind to antigens. Their constant region define how they interact with body defenses and are divided into five classes on the basis of their shape:IgG, IgA, IgM, IgD, IgE. IgG antibodies are the most common. IgE antibodies are responsible for the allergic response. Formation of an antibody-antigen complex marks a cell for destruction by phagocytes or complement.
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