Human secretory immunoglobulin M. An immunochemical and immunohistochemical study

IgM was purified from normal human colostrum and salivary secretions devoid of IgA. The isolated immunoglobulin was found to contain J chain, and showed a slight affinity for free secretory component (SC). In the secretions IgM was saturated with SC, but only 60--70 per cent of the molecules had retained the component after purification. Moreover, the I determinant of SC was much more exposed in secretory IgM than in secretory IgA, indicating a relatively 'loose' quaternary structure of the former immunoglobulin. Paired immunofluorescence staining demonstrated that secretory epithelial cells of colonic glands contained IgM in exactly the same distribution as IgA. IgM appearing in exocrine secretions is therefore a true secretory immunoglobulin in contrast to IgG. However, the quaternary structure of secretory IgM is less covalently stabilized than that of secretory IgA.