Human secretory component—I. Evidence for a new antigenic specificity

In the present work, we have studied the antigenically reactive regions of S.C. with immune sera of well-defined specificity. Immunochemical analyses conducted with an I.S. containing only precipitating antibodies against a hidden S.C. group of determinants, unravels the existence of a new antigenic specificity that we chose to call provisionally “R” (from mercapto-resistant). R was shown to be completely independent from Brandtzaeg's I and A₁ by its resistance to strong reduction and to be distinct from A₂ by the fact that anti-R antibodies do not precipitate sIgA or tryptic S.C.

The experiments described show that R occurs unaltered on spontaneously degraded S.C. as well as on the 35.000 daltons fragment from peptic digestion.

By crossed passive hemagglutination inhibition two distinct parts of the R specificity were found to occur on sIgA and tryptic S.C. respectively.

The R specificity could be located on the S.C. near the site(s) where this molecule is fixed on IgA.