Recognition that IgG consists actually of four subclasses has opened up a vast amount of knowledge regarding the biology of each subclass. Of particular interest has been the recognition that various antibodies are restricted to some subclasses. The possible role of IgG4 in immunization, desensitization, and allergy is being determined. IgG subclass deficiencies are being increasingly recognized, especially in association with infection. These patients can now be effectively treated.