Secretory component (SC): preferential binding to heavy (greater than 11S) IgA polymers and IgM in serum, in contrast to predominance of 11S and free SC forms in secretions.

The origin and molecular form(s) of the small amounts of secretory component (SC) found in serum are but poorly known. Using sensitive immunoradiometric assays for SC, we studied its molecular distribution after ultracentrifugation of native sera containing various proportions of IgA and IgM, with SC levels (sIgA equivalents) ranging from 4 to 25 micrograms/ml. Large proportions of SC were found bound to IgM and IgA polymers heavier than the classical 11S sIgA, in contrast to saliva and bile where SC was found mainly in the 11S region and in the free form. A comparative study of the ultracentrifugal distribution of labelled free SC, incubated in vitro with sera containing various proportions of IgA and IgM, yielded similar results, indicating a direct relationship between the proportion of IgM-bound SC and the IgM concentration. Free SC in serum was only found in the in vitro experiments. The data suggest that the epithelial cells synthesizing SC not only release (secrete?) SC as pre-assembled SC-Ig complexes in lymph or plasma, but also directly release some SC in free form.