Enumeration of alloreactive helper T lymphocytes which cooperate with cytolytic T lymphocytes

The number of alloreactive helper T lymphocytes (HTL) which cooperate with cytolytic T lymphocytes have been quantified in a sensitive limit dilution system. Approximately 1 in 330 splenic T lymphocytes developed helper activity in response to alloantigen of a single haplotype. Most HTL appeared to cooperate nonspecifically with the cytolytic cells. A significant number of alloreactive HTL precursors recognized H‐2 K or D alloantigen, but roughly 6‐fold more HTL precursors responded to H‐2 I, and roughly 20‐fold more responded to non‐H‐2, presumably Mls, alloantigen. These results support the hypothesis that alloantigen encoded by the H‐2 I and non‐H‐2 Mls regions preferentially stimulate helper T lymphocytes while alloantigens encoded by the H‐2 K and D regions preferentially stimulate cytolytic T lymphocytes.