The Human Secretory Immune System Shows Striking Heterogeneity with Regard to Involvement of J Chain-Positive IgD Immunocytes

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Abstract
Although gland-associated immunocyte populations normally contain a preponderance (67 to 91%) of IgA-producing cells, the proportion of the other immunocyte classes varies much among different glandular sites and from case to case. Especially notable is the relatively high percentage of IgM cells (18%), and the virtual absence of IgD cells in jejunal mucosa. Conversely, IgD-producing immunocytes amount, on the average, to 2.5% in parotid gland and to 9 to 10% in lacrimal and nasal glands. These differences were amplified in patients who completely lacked IgA cells; their gastro-intestinal mucosa contained a preponderance (56 to 91%) of IgM-producing cells, whereas the comparable specimens of nasal, lacrimal, and parotid glands contained mainly IgD-producing cells (51 to 82%) along with a smaller proportion of IgM cells (7 to 21%). The number of IgG cells was within the range of 9 to 41%, regardless of the glandular site, but amounted to 98% in a nonglandular mucosal specimen (gingiva) available from one of the patients. The total number of Ig-producing cells per glandular tissue unit (immunocyte density) was in the IgA-deficient patients generally within the ranges observed for control subjects containing normal local IgA-cell populations. It is concluded that an IgA-producing capacity of B cells is no requirement for their homing to glandular sites and subsequent local maturation. The results, instead, indicate that the usual predominance of IgA-producing immunocytes found in such sites merely reflects normal differentiation pathways taken by fairly immature B cells. The prominent J chain-synthesizing capacity shown by IgD- and even by IgG-producing cells accumulating in glandular sites in IgA deficiency most likely reflects that they belong to B cell clones in an early phase of differentiation, as has previously been suggested for IgA precursor cells homing to such sites.