An adoptive transfer system is described to measure serum helper activity in the primary antibody response to sheep red blood cells (SRBC). Mice injected with a high dose of cyclophosphamide and reconstituted with rabbit anti-thymocyte serum-treated spleen cells were used as recipients. Serum obtained 9 hr after ip injection of normal mice with 2 × 108 SRBC (S(SRBC)) injected i.v. in the recipients caused a significant enhancement of the antibody response to 2 × 107 SRBC. The serum helper activity was not generated in thymectomized animals and could be absorbed from S(SRBC) by normal and formalinized SRBC. The SRBC-specific serum helper activity (SSHA) is heat labile (30 min 56 °C) and shows allogeneic restriction. Another test system described in literature for measuring T-cell help in vivo was less suited to measure SSHA in the response to 2 × 107 SRBC. A system using normal mice injected with 105 SRBC for determining specific immune response-enhancing factor (SIREF), demonstrated SIREF activity in S(SRBC). It did, however, not measure SSHA, as absorption of S(SRBC) with formalinized SRBC did not abolish the activity in that system
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