Although secretory immunoglobulin A (IgA) is important in mucosal immunity. selective IgA deficiency is the most common primary immunodeficiency of humans. In most cases this defect is not associated with any illness. The reasons for this are unknown, but other immunological compensations might provide sufficient or complete restitution. Alternatively, it is possible that IgA deficiency alone may not predispose to disease, but additional immunological abnormalities might be present in symptomatic individuals. Some IgA-deficient individuals have a reduced antibody response to immunizations (even with normal IgG and IgM levels) and others have deficient responses to bacterial polysaccharides when IgG subclass levels are normal. The physiological role of IgA, the frequency and causes of IgA deficiency, the diseases associated with its absence, and current limited understanding of the pathogenesis of selective IgA deficiency will be reviewed.