Abstract
When effector cells from most individuals are tested directly against a number of target cells for cell‐mediated cytotoxicity, most of the reactivity is non‐selective. This apparent non‐selectiveness is a result of specific activity against multiple antigens on target cells by each effector suspension. The specific reactions can be demonstrated through competitive inhibition of natural cell‐mediated cytotoxicity. The specific inhibition of cytotoxicity by competitor cells directed our attention toward the determination of specificity to the effector cell surface. Natural cell‐mediated cytotoxicity was decreased when effector cells were treated with proteolytic enzymes or low pH and regained when these cells were incubated in serum, suggesting an antibody on cell surfaces of the effector cell. Reconstitution of trypsinized effector cells with absorbed serum resulted in selective loss of cytotoxicity against the absorbing cell. To determine the role of the effector cell and the role of antibodies in natural cell‐mediated cytotoxicity, effector cells from two different persons were reconstituted in the same absorbed sera. The selective pattern achieved was similar. When effector cells from the same individual were reconstituted with two different sera absorbed in the same way, the selective pattern achieved showed greater differences. We concluded that natural antibodies loosely attached to effector cells provided the specificity of natural cell‐mediated cytotoxicity.