Isotypes are epitopes encoded by genes identical in all individuals of a
same species. The IG and TR chain epitopes can be localized on the constant domains or on the variable domains.
Isotypes of the constant domain are a criteria of the identification of the IG or TR chain types.
This does not exclude that these same IG or TR chain be characterised by
different isotypes of the variable domain.An allele is a polymorphic variant of a gene.