Natural killer cells or NK cells belong to the cytotoxic lymphocytes and are an important part of the innate immune system. NK cells play a similar role to cytotoxic T cells in the adaptive immune response. NK cells provide rapid responses to virus-infected cells. The response time is about 3 days after infection. NK cells can also recognize and approach tumor formation and are unique in their ability to identify stressed cells even in the absence of antibodies and MHC molecules. This enables a rapid immune response. They have been called "natural killers" because they do not need to be activated to kill cells that lack MHC class 1 "self" markers. This makes the NK cell so important because harmful cells that lack MHC I markers cannot be recognized and destroyed by other immune cells, such as T lymphocyte cells. In contrast to NKT cells (CD3+56+), NK cells express neither T cell antigen receptors (TCR) nor the pan-T marker CD3. They usually express the surface markers CD16 (FcγRIII) and CD56 I in humans and NK1.1 or NK1.2 in mice.