IGHG3 and respiratory tract infectious disorder: Total serum IgG levels may vary considerably between healthy adults, but the proportion of each subclass is maintained within a relatively narrow range, by which the numeric designations of the subclasses were made: IgGl, 60–65%; IgG2, 20–25%; IgG3, 5–10%; IgG4, 3–6% [23], additionally each IgG subclass has an individual pattern of development with IgG1 and IgG3 attaining adult levels at an earlier age than IgG2 and IgG4, which is one of the reasons for higher susceptibility of infants to respiratory infections.