Lack of the G2m(n) Allotype in IgG Subclass Deficiency, in IgG2 Deficiency Together with Lack of G1m(a) and G3m(g), and in IgG3 Deficiency Together with Lack of G1m(f) and G3m(b)

Lack of G2m(n) was demonstrated in both IgG2‐deficient and IgG3‐deficient Caucasian patients. Lack of G2m(n) or G2m(″,″) was found together with homozygosity for both Glm and G3m allotypes as the dominant finding, i.e. for IgG2‐deficient patients together with Glm (f,f) and G3m(b,b), constituting the Gm(f,″,b) phenotype, and for IgG3‐deficient patients together with Glm(a,a) and G3m(g,g), constituting the Gm(a,″,g) phenotype. The group with IgG2 deficiency and the selected patients with the Gm(f,″,b) phenotype expressed characteristically very low or undetectable IgG4, significantly increased IgG3, and normal IgGl. The group with IgG3 deficiency and the selected patients with the phenotype Gm(a,″,g) expressed instead normal IgG4 and nearly normal IgG2 and IgGl levels. The lack of G2m(n) together with lack of one or the other of the alternative Glm genes and corresponding G3m genes give different IgG2 levels and different IgG subclass patterns. The frequency of Glm allotypes and corresponding G3m allotypes also deviated significantly when the IgG2 deficiency and IgG3 deficiency groups were compared with each other. Most IgG subclass‐deficient patients are homozygous in the Gm system and lack genetic variants in the three IgG subclasses, IgGl, IgG2, and IgG3.