Decreased circulating IgG concentration Decreased circulating IgG subclass concentration 2018-11-22T14:26:10Z Decreased circulating IgG subclass level IgG subclass deficiency is defined as a reduction in 1 or more subclasses of IgG (IgG1 and IgG2 in particular) by more than 2 SDs below the mean for age-matched controls. Deficiencies in IgG1 or IgG2 are more likely to cause low serum IgG levels because these are the major components. IgG1 levels stabilize by the age of 5 years; however, adult levels of IgG2, IgG3, and IgG4 may not be achieved until adolescence. Thus, the interpretation of subclass levels is additionally problematic because levels of each subclass increase at different rates during development. Decreased IgG subclass level in blood A reduction below the normal concentration of a subclass of immunoglobulin G (IgG) in the blood.