allotypes

Allotypes are genetically determined antigenic variants of proteins that differ among individuals of the same species. In immunology, the term most often refers to polymorphic determinants on antibodies but can apply to other serum proteins. Allotypes are encoded by different alleles of the same gene and are inherited in a codominant fashion, so a person may express two different alleles. They are usually located in the constant regions of immunoglobulin chains and are detected by specific alloantibodies raised in individuals lacking that variant.

In humans, well-characterized allotype systems include Gm and GM allotypes on immunoglobulin G heavy chains and Km allotypes on kappa light chains; analogous systems exist for other immunoglobulin classes and other species.

Detection methods include serological assays using antisera against known allotypes, and now genetic sequencing of constant-region loci to determine allotype constitution. Allotypes have been used in population genetics to study ancestry, kinship, and evolutionary processes; they have also played a role in transfusion medicine and transplantation, because alloimmunization to non-self allotypes can occur after exposure through blood products or during pregnancy.

Functionally, allotypes usually pertain to the constant regions and do not generally alter the antigen-binding specificity of antibodies, though they can influence interactions with Fc receptors or complement in some contexts and they are the basis for alloimmune responses when confronted with foreign allotypes.