Comment

Immunoglobulin A (IgA) is by far the most abundant immunoglobulin in humans.1 It is also the most heterogeneous. Serum IgA is mainly monomeric. It is produced by B lymphocytes in the bone marrow and in some lymphoid organs. Humans produce as much serum IgA as they do IgG. Most of the IgA in mammals is, however, found in mucosal secretions. Secretory IgA is dimeric or polymeric IgA associated with J chain and with secretory component, a part of the receptor involved in the secretion process.

Recent research2 has suggested that secretory IgA in the gut comes from two sources. Approximately 75% is from B2 lymphocytes in organised germinal centres of mucosal lymphoid tissues such as Peyer's patches. This IgA production is T lymphocyte dependent. A second source, possibly contributing around 25% of the secretory IgA, is produced by B1 lymphocytes that develop in the peritoneal cavity and are distributed diffusely in the intestinal lamina propria. This IgA may represent a primitive T lymphocyte independent source of IgA recognising commensal bacteria. Given the abundance and complexity of IgA, it is difficult to envisage that it does not perform important functions in health and disease.

Identification and characterisation of a leucocyte Fc receptor for IgA (FcαR, CD89) present on neutrophils, eosinophils, and monocytes demonstrated an active role for IgA in immunity. Many studies have now shown …