Human IgG allotypes co-occurring in more than one IgG subclass.

Inheritance of an excess of immunoglobulin allotypes in one haplotype was encountered which could not be explained by the assumption of a duplicated locus. The surplus of allotypes was related to markers on the CH3 domain of gamma 3 chains. Two such cases were investigated extensively. The IgG3 molecules were isolated by gel filtration and by absorption on protein A. Only the usual combination of allotypes appeared to be present on the IgG3 molecules. The supernumerary markers were found in one case on IgG2 molecules and in the other case on IgG1 molecules. This followed from investigations of eluates after separation of the subclasses by immune absorptions. A hypothesis was proposed to explain these events by mutation of a particular position of an otherwise homologous stretch of gamma-subclass DNA.