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. 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 are unique, however, as they have the ability to recognize stressed cells in the absence of
antibodies and MHC, allowing for a much faster immune reaction. They were named "natural killers" because of the initial notion that they do not require activation to kill cells that are missing "self" markers of MHC class 1. This role is especially important because harmful cells that are missing MHC I markers cannot be detected and destroyed by other immune cells, such as T lymphocyte cells.