Selective IgA Deficiency is the severe deficiency or total absence of the IgA class of immunoglobulins in the blood serum and secretions. There are five types (classes) of immunoglobulins or antibodies in the blood: IgG, IgA, IgM, IgD, and IgE. The immunoglobulin class present in the largest amount in blood is IgG, followed by IgM and IgA. IgD and IgE are present in very small amounts in the blood.
Of these immunoglobulin classes, it is primarily IgM and IgG that protect the bloodstream, body tissues, and internal organs from infection. It is also important that the body is protected at surfaces that come in close contact with the environment. These sites are the mucosal surfaces: the mouth and nose, the throat, the airways within the lung, the gastrointestinal tract, the eyes, and the genitalia. The IgA antibodies (which are transported in secretions to mucosal surfaces) play a major role in protecting mucosal surfaces from infection. IgG, IgM and IgE antibodies are also found in secretions at mucosal surfaces, but not in the same amount as the IgA antibody. This is why IgA is known as the secretory antibody. If our mucosal surfaces were spread out they would cover any area equal to one and one half tennis courts, so the importance of IgA in protecting our mucosal surfaces cannot be overstated.
IgA has some special chemical characteristics. It is present in secretions as two antibody molecules attached by a component called the J chain (“J” for “joining”) (see chapter on Immune System). In order for these antibodies to be secreted, they must also be attached to another molecule called the secretory piece. The IgA unit that protects the mucosal surfaces is actually composed of two IgA molecules joined by the J chain and attached to the secretory piece.
Individuals with Selective IgA Deficiency do not produce IgA. They do, however, produce all the other immunoglobulin classes. In addition, the function of their T-lymphocytes, phagocytic cells and complement system are normal or near normal. Hence, this condition is known as “Selective” IgA Deficiency.
The cause or causes of Selective IgA Deficiency are unknown. It is likely that there are a variety of causes for Selective IgA Deficiency and that the cause may differ from patient to patient. Individuals with Selective IgA Deficiency have B-lymphocytes that appear to be normal, but do not mature into IgA producing plasma cells.
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