Natural killer (NK) cells. Immunofluorescent light micrograph of human natural killer cells (centre, with branched cytoplasm) attacking cultured cancer cells (unstained except for large blue nuclei). Granules of cytotoxin in the NK cells' cytoplasm are green. Natural killer cells are lymphocytes which can destroy virus-infected cells & tumour cells in the absence of antibody. On contact with the surface of a tumour 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.
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