Ans. is 'b' i.e., Epitope Epitope . Also known as "antigenic determinant". . It is antibody binding site of antigen. . It is the smallest unit of antigenicity. . Antigenic determinants is composed of structures on the surface of molecules and can be constructed in two ways: 1. Within a single segment of primary sequence 4. Sequential epitopes 2. Assembled from residues far apa in the primary 3. Conformational epitopes sequence but brought together on the surface by the folding of the molecule into its native conformation . The majority of antigenic structures recognized by antibodies depend on the teiary configuration of the antigen (conformational), while T cell epitopes are defined by the primary structure (sequential). Paratope . The pa of the antibody or T cell receptor that interact with the epitope. "The idiotype is defined as the specific region of the fab poion of the 1g molecules to which antigen binds".