The genetic and biochemical basis for the inclusion of T cell alloreactivity within the immunological repertoire of T helper (Tn) 1 lymphocytes remains a perplexing problem (1, 2). Recent evidence (3-9) suggests that the ability of Tn cells to bind to molecular complexes formed on antigen-presenting ceils between a nominal antigen and either self or nonself (allo) Ia molecules regulates their d...