During evaluation of a patient for possible immunoglobulin deficiency, quantification of IgG, IgA and IgM are usually performed. However, normal levels of these immunoglobulins will not rule out some forms of immunodeficiency. Quantifying the subclasses of IgG may help determine the immune status of a patient with normal total IgG. It is known that some subclasses preferentially react to particular antigens. IgG2 antibodies react to carbohydrate antigens and polysaccharides, including bacterial capsules. IgG4 antibodies react to allergens and parasitic antigens. The latter antibody may represent a non-IgE antibody which could sensitize basophils for release of histamine with production of allergic symptoms. When IgG2 or IgG4 deficiency accompanies IgA deficiency, the patients are particularly prone to suffer from infections. Patients with ataxia telangiectasia can also have deficiency of IgG subclasses. While not a primary screening test, quantification of IgG subclasses can be useful in patients with IgA deficiency, and in individuals with normal levels of IgG but an apparent poor response to pyogenic bacterial infections. The diagnosis is quite likely when IgG and/or IgA levels are low. Patients with these deficiencies usually respond well to gamma globulin replacement therapy. IgG3 has high antiviral activity.