Materials and MethodsAnimals. BDF~ (C57BL/6 female × DBA/2 male F~) and A/J mice, 8-to 10-wk of age (The Jackson Laboratory, Bar Harbor, Maine) were used in this study. Animals to be used as the source of helper T cells were carrier-primed by intravenous injection of sheep erythrocytes (SRBC) ~ (Colorado Serum Co., Denver, Colo.; 0.2 ml of 0.01% suspension) 7 days before sacrifice (10). Spleen cell suspensions from primed animals were irradiated (1500 R; ~ZTcesium source) to remove B-cell activity before cultivation.Antigens. Human gamma globulin (Cohn fraction II, Sigma Chemical Co., St. Louis, Mo.) was haptenated by using 2, 4, 6, trinitrobenzene sulfonic acid (TNBS, J. T. Baker Chemical *

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