Significance of class 1 and class 2 major histocompatibility complex antigens: help to allogeneic K and D antigens does not involve I recognition.

The significance of the dichotomy of MHC antigens was investigated. It was determined that primed helpers directed against allogeneic class 1 antigens (K and D in the mouse) were not restricted to syngeneic class 2 antigens (I in the mouse). Furthermore, the generation of helper T cell-replacing factor by stimulation with K or D antigens was not blocked by Ia antibodies that were effective in blocking the same factors generated in response to I region MHC antigens. It is concluded that class 2 antigens are not recognized during the response of helpers to class 1 antigens, and it is suggested that class 1 and class 2 molecules can perform reciprocal functions.