Secretory Component Deficiency: A Disorder of the IgA Immune System

We studied a 15-year-old boy with chronic intestinal candidiasis who had normal serum IgA levels without IgA in his secretions. There was an elevated number of peripheral blood lymphocytes bearing surface IgA. In addition, the lymphocytes cultured in vitro with pokeweed mitogen produced IgA as well as other immunoglobulins. Despite this evidence of normal IgA synthetic capacity, the patient had greatly diminished levels of IgA in the saliva and jejunal fluid, and, as estimated by ¹⁴C-L-leucine incorporation, could not synthesize IgA locally at intestinal-mucosal sites. Finally, the patient had no detectable free secretory component in saliva or jejunal fluid in contrast to normal persons and to patients with IgA deficiency. The basis of this disorder is probably a defect in the homing of IgA precursor cells to secretory sites or in the selective proliferation/differentiation of IgA cells at such sites. (N Engl J Med 294:351–356, 1976) In the usual patient with selective IgA deficiency there is absence of both serum and secretory IgA.¹In the patient described below IgA was present in normal concentration in the serum but was virtually absent from the secretions. The origin of this disorder is strikingly different from that of ordinary selective IgA deficiency, and, as such, this case offers instructive insights into the physiology of the IgA system. Case Report A 15-year-old boy had been the product of a normal pregnancy. The birth weight was 4.1 kg, and the neonatal period was uneventful. Diarrhea developed upon weaning at 3 1/2.