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CELL‐MEDIATED IMMUNITY TO H‐2 ANTIGENS: Characteristics of the Effector Cells as Detected in the Microcytotoxicity Assay

Cell-mediated immune reactivity against murine histocompatibility antigens was compared by using both the microcytotoxicity assay (MCA) and the 51chromium release assay (CRA). In both assay systems the data obtained showed T cell-mediated reactivity. In the CRA only T lymphocytes were detectable, which were cytotoxic. In contrast, in the MCA reactivity of nonlytic secondary T cells (NLST) was also detected. NLST acquired their MCA-reactivity within the first 24 hr of the 48-hr test period of the MCA. Circumstantial evidence is provided suggesting that MCA reactivity is a product of two different processes, namely, antigen-specific restimulation of immune T cells and shedding of cytotoxicity inhibiting factors.