suured granulaarsed tsütotoksilised lümfotsüüdid, mille peamiseks ülesandeks on ära tunda ja hävitada makroorganismi ebanormaalseks muutunud rakke (peamiselt kasvajarakke ja viirusega infitseeritud rakke), Natural killer cells or NK cells are a type of cytotoxic lymphocyte critical to the innate immune system. The role NK cells play is analogous to that of cytotoxic T cells in the vertebrate adaptive immune response. Typically, immune cells detect major histocompatibility complex (MHC) presented on infected cell surfaces, triggering cytokine release, causing lysis or apoptosis. NK cells provide rapid responses to viral-infected cells and respond to tumor formation