Evidence for three types of human cytotoxic lymphocyte

Natural killer (NK) cells lyse a broad spectrum of tumor-cell lines and virus-infected cells, they are MHC unrestricted and they are detectable in unimmunized subjects: cytotoxic T cells (CTL) have fine antigen specificity, are MHC restricted and are usually detectable only after they multiply on exposure to antigen. Here Lewis Lanier and Joseph Phillips argue that these functional criteria do not adequately distinguish types of cytotoxic cell. They propose a classification based on recognition structure, which identifies three distinct effector cells.