Antibody Structure. Adapted from people.cryst.bbk.ac.uk/~ubcg07s/gifs/igG.gif. . Variable domain of Antibodies. . Humanization of Antibodies. Epitopes. . The actual portions or fragments of an antigen that react with receptorson B-lymphocytes and T-lymphocytes, as well as free antibody molecules,are called epitopes or antigenic determinants. The size of an epitope isgenerally thought to be equivalent to 5-15 amino acids or 3-4 sugar residues..