antigenic Definition

any substance (such as an immunogen or a hapten) foreign to the body that evokes an immune response either alone or after forming a complex with a larger molecule (such as a protein) and that is capable of binding with a product (such as an antibody or T cell) of the immune response

epitope

a molecular region on the surface of an antigen capable of eliciting an immune response and of combining with the specific antibody produced by such a response —called also determinant, antigenic determinant