Studies on the antigenic structure of human and hog intrinsic factors.

The antigenic interrelationships of hog and human intrinsic factors (IF) were investigated using four different antibodies to IF. Rabbit antisera to hog IF, goat antisera to hog IF, rabbit antisera to human IF and human autoantibodies to IF were tested and quantitated for blocking and binding antibodies against both types of IF. The blocking antibodies interacted with both the homologous (immunizing) and the heterologous IF, although the antibody activity was greatest against the homologous IF. Only the rabbit antiserum to human IF blocked all B combining sites on the homologous as well as the heterologous IF. Binding antibodies also reacted greatest with the homologous IF, and all, except for the goat anti-hog IF, reacted to a lesser extent with the heterologous IF. These data indicate that human and hog IF have similar, although not identical antigenic structures; both contain two major antigenic determinants, one located at the B combining site and a second which is remote from this site. Each major determinant appeared to consist of several antigens, some of which are species similar. Moreover, each antigenic determinant elicited a distinct immune response. Following immunization, blocking antibodies were detected only in the IgG fraction, whereas both IgG and IgM immunoglobulins contained binding antibody activity. Thus, the concept of two separate antigens on the IF molecule is supported by the differences in biological activity and in the immunoglobulin class of antibodies which they elicit.