Immunoglobulins (Ig) are glycoprotein molecules that function as antibodies. Since antibodies are present in the bloodstream or bound to cell membranes, they are considered part of the humoral immune system. The humoral system is mediated by secreted antibodies.
Immunoglobulin G (IgG) antibodies are the smallest, but most abundant antibodies in the body, making up 75-80% of all the antibodies in the body. They are present in all body fluids. The IgG antibodies are considered the most important antibodies for fighting against bacterial and viral infections, and they are the only antibodies that can cross the placenta during pregnancy.
IgG isotypes are associated with complement fixation (immune response in which an antigen-antibody combination inactivates a complement) opsonization (process by which antigens are altered so that they are more efficiently engulfed and destroyed by immune cells), fixation to macrophages and membrane transport.
There are four subclasses of the IgG class of antibodies -
IgG1, IgG2, IgG3 and IgG4. As the antibody-producing B-cell matures, it can switch from one subclass to another.
In healthy individuals 60-70% of IgG antibodies in the bloodstream are IgG1, 20-30% are IgG2, 5-8% are IgG3 and 1-3% are IgG4. The levels of IgG subclasses in the bloodstream vary with age. IgG1 and IgG3 reach normal adult levels when an individual is about five to seven years old, while IgG2 and IgG4 levels rise more slowly, reaching adult levels at about 10 years of age. In young children, the ability to make antibodies to bacteria (typically antibodies of the IgG2 subclass) develops more slowly than the ability to make antibodies to proteins.
IgG subclass deficiencies occur when the B-cells do not mature properly, and they are unable to switch IgG subclasses. This causes an imbalance of the IgG subclasses, with one or more subclasses being deficient. The overall level of IgG can be normal, but individual subclass levels may be higher or lower than normal. IgG2 deficiency is the most common subclass deficiency in early childhood, while IgG3 deficiency is the most common in adults.
Individuals who have IgG subclass deficiency may have increased susceptibility to infections, such as ear infections, sinusitis, bronchitis and pneumonia.
The incidence of IgG subclass deficiency remains unknown. No epidemiological studies have been performed to determine its prevalence in any specific population.
The prognosis for patients with IgG subclass deficiency is generally good. Many children appear to outgrow their deficiencies once they reach adulthood. While there is currently no cure for IgG subclass deficiency, treatment can help prevent infections commonly associated with the disorder. For those patients who experience persistent deficiencies, antibiotics, immunizations and gammaglobulin replacement therapy may help prevent serious infections and the development of impaired lung function, hearing loss or other injuries caused by infections.
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