Natural killer (NK) cells. Immunofluorescent light micrograph of human natural killer cells. Granules of cytotoxin in the NK cells' cytoplasm are green, nuclei are blue and cytoplasm red. Natural killer cells are lymphocytes (distinct from B- or T- lymphocytes) which can destroy virus-infected cells & tumour cells in the absence of antibody. On contact with the surface of a tumour cell or a virus-infected cell a NK cell recognizes certain antigens, which activate its cell-killing mechanism. Immunofluorescence is a staining technique which uses antibodies to attach fluorescent dyes to specific tissues and molecules within the cell. Magnification: x312 at 35mm size.