172 THE EFFECT OF ISOANTISERUM ON LMC Preparation of Sensitized Spleen Cells

Lymphocytes derived from animals immunized against alloantigens exhibit in vitro cytotoxicity for target cells carrying the respective isoantigen (1). This cytotoxic activity is mediated by theta-bearing (thymus-derived) lymphocytes (T cells) 1 (2, 3). The accumulated evidence indicates that the cellular immune response to an isoantigen or tumor cell exists concomitantly with isoantibodies secreted by bone marrowor bursa-derived cells (B cells) that interfere with the in vitro cytotoxic activity of immune T cells (1, 4). I t is now possible to quantitate the cytotoxic activity of sensitized lymphocytes through the use of 51Cr release from labeled target cells, a procedure developed by Brunner et al. (1) and Holm and Perlman (5).