It is thought that NK cells participate in the control of nascent tumors and, perhaps, of metastatic growth. CTLs are considered to be the main effector cells in the rejection of transplanted tumors and of tumors caused by DNA viruses. It is becoming increasingly evident that CTLs can kill target cells via two distinct pathways. LAK cells are a subset of null lymphocytes distinct from NK cells and CTLs, and they have antitumor activity. Finally, macrophages that have been activated directly by a variety of stimulants or indirectly by the products of antigen-stimulated T cells can kill tumor cells in a manner that is not antigen dependent and not MHC restricted.
Activated macrophages appear to decrease the growth rate of tumors that they infiltrate.
These immune effector mechanisms are thought to be far less important in vivo than the tumoricidal action of NK cells, LAK cells, CTLs, and activated macrophages.