A human serum immunoglobulin with specificity for certain homologous target cells, which induces target cell damage by normal human lymphocytes.

A factor has been found in a number of human sera which renders a polyploid strain of human liver cells, Chang cells, susceptible to damage by non-immune human lymphocytes. Sera possessing this factor are referred to as Factor Containing Sera (FCS). Such damage is assessed quantitatively by release of radioactive chromium from target cells. This factor has the chemical properties of IgG and can be absorbed out on Chang cells. Its specificity has been shown to be for Chang cells and not for human lymphocytes. Other homologous and heterologous target cells tested were not affected by this factor. The factor has not been shown to have any effect on Chang cell viability by itself, even in the presence of complement. Factors which inhibit target cell damage are shown to coexist with the factor which induces non-immune lymphocyte damage of Chang cells. The possible origin of this factor is discussed as is the role in immune reactions of target cell specific antibody which renders such cells susceptible to damage by non-immune lymphocytes.