Allotype (from Greek allotypos = differently shaped). Structural differences between genetically non-identical individuals of the same species, determined by different alleles.
These differences on the surface of some molecules (e.g. blood groups, histocompatibility antigens, immunoglobulins), which can be identified as phenotypic, are immunogenic for the carrier of another allotype. Allotypes play an important role e.g. in immunoglobulins. There they are caused by differences in the constant parts of the heavy and light chain.