SummaryJejunal biopsies obtained from 45 children referred for a variety of clinical conditions were examined histologically and by standard immunofluorescence methods. The number of plasma cells was recorded per “mucosal tissue unit.” The results showed that most of the immunoglobulin A (IgA) counts in infants under 3 yr with normal mucosa were low and that there was a rise in the number of IgA plasma cells starting after this age. Despite this, under pathologic conditions (partial villous atrophy with increased inflammatory cell infiltration - PVA) a significant rise in IgA plasma cells occurred in all age groups. Patients with coeliac disease tended to show the highest IgA as well as IgM plasma cell counts. In all other patients, IgM counts did not change significantly with age or histologic abnormalities.SpeculationChildren in the first few months of life are as capable as older children of increasing the number of IgA plasma cells in their jejunal mucosa. However, though immunocompetent, their gut, on first contact, will be lacking in primary immune responses to the various antigens, and, therefore, may be relatively unprotected.