An antigenic determinant, a site on the antigen that the immune system responds to by making antibody, can frequently be one unique structure on the antigen. In hen egg white lysozyme, a glutamine at position 121 (Gln 121) protrudes away from the antigen surface. In this view, Gln 121 is circled. The antibody is not shown. The following images show how this feature is important for the formation of a high affinity antibody-antigen interactions.