Cytotoxicity is the quality of being poisonous to cellss. This can be a chemical substance or an immune cell.

Antibody-dependent cell-mediated cytotoxicity (ADCC) describes the cell-killing ability of certain lymphocytes, which requires the target cell being marked by an antibody. Lymphocyte-mediated cytotoxicity, on the other hand, does not have to be mediated by antibodies.

Three groups of cytotoxic T lymphocytes are distingushed: antigenic (immunized) T lymphocytes, killer cells, and natural killer cells.