High proportion of polymeric IgA in young infants' sera and independence between IgA-size and IgA-subclass distributions.

The serum IgA in young infants (6-240 days old) comprised a much larger proportion (means = 36%; range, 7.5-72%) of polymeric IgA than in normal adults (means = 13%; range, 4.7-22%), but with a normal subclass distribution. In contrast, in IgA-deficient adult serum, there was a higher proportion (means = 34%; range, 16.3-38%) of IgA2 than in normal adult serum (means = 21%; range, 5-36%), with, however, a normal proportion of polymers. The distribution of IgA subclasses in the separated monomeric and polymeric IgA fractions was similar to that in unfractionated sera. The proportions of IgA polymers and IgA2 in serum are unlinked parameters, suggesting that both do not necessarily represent mucosal contributions to blood IgA.