Quantitative studies of natural immunity to solid tumours in rats. Persistence of natural immunity throughout reproductive life, and absence of suppressor cells in infant rats.

Consideration of cell-mediated cytotoxicity as an enzyme-substrate reaction provided a theoretical and practical basis for the quantification of natural immunity to solid tumours. The natural cytotoxic activity of spleen cells from normal rats towards cultured target cells from solid tumours, measured in a 6 h chromium release assay, developed between 2 and 5 weeks after birth and, in contrast to previous reports of natural cytotoxicity against lymphoid tumour cells, showed no sign of decline up till at least 22 months of age. Both virgin and breeder (multiparous) females showed equally good maintenance of natural immunity, and the apparent specificity of this natural immunity on a panel of four target cells did not change between 8 weeks and 18 months of age. The low level of natural immunity in infant spleen, and also in adult lymph nodes and thymus, was not caused by suppressor cells but rather by an absence of appropriate effector cells.