Selective IgA Deficiency is the most common primary immunodeficiency disease (PIDD). People with this disorder have absent levels of a blood protein called immunoglobulin A (IgA). IgA protects against infections of the mucous membranes lining the mouth, airways and digestive tract.

Although individuals with Selective IgA Deficiency do not produce IgA, they do produce all the other kinds of immunoglobulin. This is why many people with IgA deficiency appear healthy or only have mild illness such as gastrointestinal infections.

A common problem in IgA deficiency is susceptibility to infections. A second major problem in IgA deficiency is increased occurrence of autoimmune diseases. Also, many people with Selective IgA Deficiency also have allergies or asthma.