IgG subclass deficiencies affect only IgG subclasses (usually IgG2 or IgG3), with normal total IgG and IgM immunoglobulins and other components of the immune system being at normal levels. In young children, the ability to make IgG2 antibodies to the polysaccharide coatings of bacteria develops more slowly than the ability to make antibodies to proteins. For example, IgG1 and IgG3 reach normal adult levels by 5-7 years of age while IgG2 and IgG4 levels rise more slowly, reaching adult levels at about 10 years of age. The amount of the different IgG subclasses present in the bloodstream varies with age. In contrast, IgG2 antibodies are predominantly against the polysaccharide (complex sugar) coating (capsule) of certain disease-producing bacteria (such as, Streptococcus pneumoniae and Haemophilus influenzae). For example, IgG1 and IgG3 subclasses are rich in antibodies against proteins such as the toxins produced by the diphtheria and tetanus bacteria, as well as antibodies against viral proteins. While all the IgG subclasses contain antibodies to components of many disease-causing bacteria and viruses, each subclass serves a slightly different function in protecting the body against infection. Patients with persistently low levels of one or two IgG subclasses and a normal total IgG level have a selective IgG subclass deficiency. These are designated IgG1, IgG2, IgG3 and IgG4. The IgG class of antibodies is composed of four different subtypes of IgG molecules called the IgG subclasses. (See chapter titled “ The Immune System and Primary Immunodeficiency Diseases.”) Most of the antibodies in the blood and the fluid that surround the tissues and cells of the body are of the IgG class. There are five types or classes of immunoglobulin: IgG, IgA, IgM, IgD and IgE. Definition of IgG Subclass DeficiencyĪntibodies are also called immunoglobulins. A subclass deficiency needs to be considered and looked for only under special circumstances discussed in this chapter. The misdiagnosis of IgG subclass deficiency as a cause of presumed immunodeficiency is common, often leading to the unnecessary long-term use of Ig replacement therapy. Although this deficiency may occasionally explain a patient’s problems with infections, IgG subclass deficiency is a controversial diagnosis and experts disagree about the importance of this finding as a cause of repeated infections. When one or more of these subclasses is persistently low and total IgG is normal, a subclass deficiency is present. IgG is a combination of four slightly different types of IgG called IgG subclasses: IgG1, IgG2, IgG3 and IgG4. This is the second most abundant circulating protein and contains long-term protective antibodies against many infectious agents.

The main immunoglobulin (Ig) in human blood is IgG.

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