An antigenic determinant, also known as an epitope, is a specific part of an antigen that is recognized and bound by an antibody or a receptor on the surface of a T cell. It is essentially the "lock" to which the immune system's "key" (the antibody or T cell receptor) specifically binds.
Topic 21.3: 21.3 The Adaptive Immune Response: T lymphocytes and Their Functional Types
Unit 21