A conformational epitope is a sequence of sub-units (usually amino acids) composing an antigen that come in direct contact with a receptor of the immune system. An antigen is any substance that the immune system can recognize as foreign. Antigens are usually proteins that are too large to bind as a whole to any receptor so only specific segments, that form the antigen, bind with a specific receptor. Such segments are called epitopes. Likewise, it is only the paratope of the receptor that comes in contact with the epitope.