A serum containing antibody activity within the MN blood group system is described. Absorption studies showed that the serum detects two new antigenic determinants that have been named Tm and Sj. The results obtained when 1,400 bloods, 500 from Negroes and 900 from Caucasians, were tested for ten antigens in the MN system -M, MI, N, S, s, U, Hu, He, Tm and Sjare presented. Unlike most of the recently described new antigens in the MN system both Tm and Sj are of fairly high frequency in Caucasians and Negroes. Calculations of gene frequencies from these data are published in an accompanying paper [7].

A serum containing antibody activity within the MN blood group system is described. Absorption studies showed that the serum detects two new antigenic determinants that have been named Tm and Sj. The results obtained when 1,400 bloods, 500 from Negroes and 900 from Caucasians, were tested for ten antigens in the MN system - M, M(1), N, S, s, U, Hu, He, Tm and Sj - are presented. Unlike most of the recently described new antigens in the MN system both Tm and Sj are of fairly high frequency in Caucasians and Negroes. Calculations of gene frequencies from these data are published in an accompanying paper [7].

In order to examine whether the H-2 haplotype of a thymus graft influences the levels of IgG3, IgG1, IgG2b, and IgG2a antibodies produced in an in vivo response to antigens under immune response (Ir) gene control, genotypic high-responder (H-2b X H-2k)F1.nude mice were grafted with thymuses from irradiated, neonatal low-responder (H-2k) or high-responder (H-2b or (H-2k X H-2b)F1) mice and immunized with (T,G)-A--L. All IgG antibody responses to (T,G)-A--L in high-responder mice were shown to be thymus dependent. The majority of F1.nude mice grafted with thymuses from high-responder haplotype donors produced high-responder levels of IgG anti-(T,G)-A--L antibodies. Conversely, the majority of F1.nude mice grafted with thymuses from low-responder haplotype donors gave low-responder phenotypic patterns. The modulation of the Ir gene phenotype was not restricted to a particular IgG isotype, but affected IgG3, IgG1, IgG2b, and IgG2a. The F1 . nude mice grafted with low-responder haplotype thymuses were able to produce IgG1, IgG3, and IgG2b antibodies to sheep erythrocytes, a thymus-dependent (TD) antigen not under overt Ir gene control. Circulating peripheral T cells were shown to be of host origin. By these criteria, the thymus grafts did enable F1.nude mice to respond to a TD antigen. These results support the concept that thymic H-2 determinants are involved in at least the selection of H-2 restricted T cell subsets, if not also the derivation of the T cell receptor repertoire for self and/or antigen recognition.