An epitope associates with which part of an antigen receptor or antibody?
a. the tail
b. the heavy-chain constant regions only
c. variable regions of a heavy chain and light chain combined
d. the light-chain constant regions only

c. Variable regions of a heavy chain and light chain combined

Explanation:

The epitope is the specific part of an antigen that an antibody recognises and binds. An antigen is basically any molecule that causes an immune response.

An antigen binding region on an antibody (sometimes called the antigen-binding fragment, or Fab) consists of one constant and one variable domain of each of the heavy and the light chain (Either side of the Y shape). An epitope interacts with both these structures. See the attachment for how it binds both.

Since it associates with both heavy and light chains, it cannot be b or d. It does not associate with the tail (the bottom of the Y shape) which is used to communicate with other members of the immune system, therefore it cannot be a.