The alloantigenic organization of RT1Aa, a class I major histocompatibility complex molecule of the rat.

Over 300 monoclonal IgG alloantibodies have been prepared against RT1Aa , the class I major histocompatibility complex molecule of the DA rat. In this study a combination of techniques is exploited to show that all these antibodies can be allocated to 9 antigenic sites which form a continuous antigenic surface, that is, no site is completely isolated from the rest. The results suggest that techniques for the identification of antigenic sites using competitive inhibition of monoclonal antibody binding are generally valid, in the sense that competition between antibodies appears most commonly to represent competition between combining sites for a structural feature of the antigenic surface. From the distribution of antibodies between sites, it is clear that the RT1Aa molecule has three immunogenic areas against which nearly all the antibodies studied were directed. Of these areas one is both antigenically complex, consisting of four closely spaced sites, and remarkably immunodominant. Antibodies directed at sites between the major areas are extremely rare.