Selective concentration of IgD class‐specific antibodies in human milk

SUMMARY The participation of human IgD class antibody in local immune responses of breast tissue was studied by analysing the sera‐to‐milk ratios of total IgD, IgM, IgA, IgG isotypes and albumin found in matched samples, and by analysing the sera‐to‐milk (S/M) ratios of IgD, IgM, IgA, IgG antibodies against Haemophilus influenzae capsular polysaccharide (PRP), phosphorylcholine, tetanus and in some cases diphtheria antigens. The study group consisted of eight women immunized during pregnancy with PRP, and control, unimmunized women. Albumin, and total IgG showed high S/M ratios. IgA had a low S/M ratio as expected, consistent with reports that IgA is locally concentrated. Total IgD and IgM isotype ratio values were intermediate between IgG and IgA suggesting they were selectively concentrated in breast fluids due to local production or transport mechanisms, or both. Ratios for specific antibodies of IgA and IgM isotypes and for total IgA and IgM isotype showed parallel data. Among the IgD antibodies, those specific for PRP and phosphorylcholine suggested a higher degree of selective concentration as compared with tetanus antigen. In the group of unimmunized women, although selective concentration of total IgD was observed, specific antibody studies were inconclusive due to the low milk IgD antibody levels encountered. The results indicate that IgD (and also IgM) may participate in local immune responses of human breast tissues and fluids; possibly influenced by the nature of the antigen, the state of immunization and the hormonal environment (pregnancy).