Papain‐resistant (f) and papain‐sensitive (g) subclasses of rabbit slgA. Allotypic specificities on different domains of the g subclass of α‐chains (αg)

Secretory IgA (sIgA) from rabbits homozygous for the f71 and g75 allotypic specificities of the f and g subclasses, respectively, was digested into four fractions: sIgA, Fc_2α, Fab_2α and Fab_α. The subclass and allotypic characteristics of each fraction were determined by quantitative radioprecipitation analyses. The f71 allotypic specificities were found predominantly in the undigested sIgA (with some in the Fab_α fragment) and the g75 allotypic specificities were in the Fc_2α and Fab_2α fractions. Previous data from our laboratory indicated that the f72 and f73 sIgA allotypes of the g subclass are resistant to cleavage and that the g74 sIgA allotype of the g subclass is cleaved by papain. The present and previously reported data support the hypothesis that resistance or sensitivity of rabbit sIgA to papain digestion is a function of the subclass rather than the individual allotypic specificities. Quantitative radioprecipitation analyses of the Fc_2α and Fab_2α fragments showed that the “g” allotypic specificities are composed of multiple determinants, some of which reside on the Fd part of the α‐chain and some of which reside on the Fc part of the α‐chain. Anti‐allotype antisera have been prepared which are specific for the allotype determinant(s) present on each of the fragments. Of special interest is the ability to serologically distinguish the C_H1 domain on the Fd fragment of a rabbit α‐chain.