An immunology term that refers to the specific portion of an antigen that stimulates an immune response, and the part of the antigen to which the antibody actually binds. So, an epitope is generally a smaller portion of a larger antigen molecule (whether protein, carbohydrate, lipids, or mixtures of the above) that is enormously important, because it determines the specificity of the antigen. The term is also synonymous with “antigenic determinant.”