[Natural killer cells--important in the immunologic defense]

Natural Killer (NK) cells are a group of lymphocytes with a characteristic morphology and function. They are abundant in blood, spleen, liver and lungs. They are distinct from both T and B lymphocytes in their circulation patterns, profile of surface antigens, receptor repertoire and the way they discriminate between self and nonself. This latter NK function can partly be explained by an array of recently characterised NK receptors that can recognise and accurately discriminate between normal and altered MHC class I determinants. The basis for this discrimination is different from that of T cells and is discussed in this article. The role of NK cells in antimicrobial defense is well demonstrated, particularly that against viruses belonging to the herpesvirus group. A case report of a patient with a selective defect in NK cells and with recurrent viral infections is described. The role of NK cells in defense against malignant cells is more circumstantial, but NK cells do possess receptors which recognise tumour cells and kill them efficiently in vitro. A receptor which can recognise determinants unique for cancer cells has recently been described.