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Antibodies, Antigens, and Epitopes

IHC is centered on the antibody/antigen relationship. For a complete understanding of IHC, some general knowledge of immunology is needed.
An antibody is any of the complex proteins produced by B-lymphocytes in response to the presence of an antigen. In IHC, antibodies are used to attach and identify antigens. Antibodies are also known as immunoglobulins. Immunoglobulins are Y-shaped proteins made up of two identical heavy chains which determine its class (IgA, IgD, IgE, IgG, or IgM) and two identical light chains, either kappa or lambda. The short arms of the Y are the areas of the antibody that attach to their specific antigen.
Antigens and Epitopes
Epitopes are areas on the antigen which allow the short arms of a Y-shaped antibody to attach and bind to that particular antigen. Antibodies may attach to more than one antigen, but are specific for only one epitope. Antigens are protein or oligosaccharide markers on the surface of cells that identify the cell as self or non-self and are capable of producing an immune response in animals or cell cultures for the production of antibodies. In IHC, the terms antigen and epitope are used interchangeably.