B10.A(5R) mice immunized with C57BL/10 spleen cells demonstrate a normal T-cell-mediated cytotoxicity to H-2Db tumor cells but they do not mount any IgG antibody response to H-2Db alloantigens. B10.A(5R) mice do show a high titered IgG response when immunized with A.BY cells, which differ at H-2Db plus non-H-2 cell surface antigens, or with B10.A(2R) cells, which differ at H-2Db, H-2Kk, and H-2Ik cell surface antigens. These findings indicate a failure of the T-helper cells to induce the switch from IgM to IgG when the H-2Db alloantigens are the only difference on the immunizing cell. In immunizing H-2d mice with congenic H-g2 cells which differ only in the H-2Db region, mice of the C57BL/10 background made only IgM antibodies whereas mice of the BALB/c background made IgG antibodies. This comparison confirms that genes separate from H-2 regulate the T-cell helper function. The genes that influence the T-cell helper function do not regulate the T-cell-mediated cytotoxicity.

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