There are four subtypes, of which IgG1 and IgG2 comprise 85% of the total. IgG1 and IgG3 fix complement best; IgG3 is hyperaggregable and effects serum viscosity disproportionately.
IgG antibody responses to certain antigens occur to a greater extent in one type of IgG subclass than another. Therefore, some patients with normal total IgG levels may have problems with pyogenic infections because they do not produce IgG2 or combinations of IgG2, IgG3, and/or IgG4. Some clinically significant IgG subclass deficiencies occur in patients who have IgA deficiency.
The four subclasses of IgG differ in the constant regions of their heavy chains. A patient may have a normal total IgG yet still have a significant decrease in one subclass. IgG1 deficiencies are associated with EBV infections, IgG2 with sinorespiratory infections and infections with encapsulated bacteria, IgG3 with sinusitis and otitis media, and IgG4 with allergies, ataxia telangiectasia, and sinorespiratory infections. See tables in individual subclass listings..