The genetics of IgG4 deficiency: Role of the immunoglobulin heavy chain constant region and HLA loci

IgG4 deficiency is very common (1/400 in the Italian population) and provides a good model for analyzing the genetic factors involved in Ig subclass deficiencies. We have previously reported an association between some immunoglobulin heavy chain constant region (IGHC) polymorphisms and the IgG4 deficiency. The associated polymorphisms spanned the region between the GP and the G4 genes. A larger sample composed of 50 healthy blood donors with IgG4 deficiency (< 0.001 g/l IgG4), not carrying homozygous gene deletions, together with 82 first‐degree relatives is now examined. The results confirmed the association of the deficiency with IGHC polymorphisms, and detected a new association with the HLA‐D locus with a strong additive effect between the two systems. However, despite these associations and a highly significant risk for IgG4 deficiency within families, close linkage with either IGHC or HLA loci was not apparent by the affected sib pair method. These findings suggest that several concomitant, possibly cooperating, genetic factors may be involved in IgG4 deficiency.