, also known as

or

, are a type of

critical to the

. The role of NK cells is analogous to that of

in the vertebrate

. NK cells provide rapid responses to

-infected cells, acting at around 3 days after

, and respond to

formation. Typically,

detect the

(MHC) presented on infected cell surfaces, triggering

release, causing

or

. NK cells are unique, however, as they have the ability to recognize stressed cells in the absence of

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.

cytotoxiclymphocyteinnate immune systemcytotoxic T cellsadaptive immune responsevirusinfectiontumorimmune cellsmajor histocompatibility complexlysisapoptosis