Effect Factor , O n e of Which Displays H - 2 - restr icted Helper Activi ty and the Other , T Cel l - Growth Factor Activity

During differentiation in a thymus, T lymphocytes learn to recognize self alloantigens determined by the H-2 major histocompatibility complex. This imposes certain genetic restrictions on lymphocyte interaction during T cell-mediated immune responses (1-3). For example, antigen-specific helper T cells acquire a binding capacity not only for a given foreign antigen but also for Ia alloantigens of the host haplotype. Our previous studies have shown that soluble allogeneie effect factors (AEF) a derived from mixed lymphocyte reaction (MLR) cultures of graft-versus-host reaction (GVHR)-activated responder T cells and either H-2(4, 5),/-region(6),/-subregion(7, 8) or Mls-locus(9) incompatible irradiated stimulator cells are genetically restricted in their helper activities. They each preferentially help primary and secondary antibody responses of B cells of the stimulator haplotype. The activity of several of these factors may be absorbed by alloantibodies reactive with I-A(5, 6) or I-J(8) subregion products of the stimulator haplotype but not of the responder haplotype. The genetic restriction of these responses may be controlled in part by the acquisition by G V H R donor and MLR responder cells of G V H R host and MLR stimulator Ia antigens (10). Thus, although it is apparent that both helper T cells and AEF helper