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Colorized scanning electron micrograph of a natural killer cell from a human donor. Natural killer 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. NK cells provide rapid responses to viral-infected cells, acting at around 3 days after infection, and respond to tumor formation. Typically, immune cells detect major histocompatibility complex (MHC) presented on infected cell surfaces, triggering cytokine release, causing lysis or apoptosis. NK cells have the ability to recognize stressed cells in the absence of antibodies and MHC, allowing for a much faster immune reaction.
