Natural killer cells or NK cells are a type of cytotoxic lymphocyte and are a important part of the innate immune system. NK cells play is similar role to that of cytotoxic T cells in the adaptive immune response. NK cells provide rapid responses to viral-infected cells. The response time is about 3 days after infection. NK cells can also recognize and approach tumor formation and are unique, as they have the ability to identify stressed cells also in the absence of antibodies and MHC molecules. That enables a fast immune reaction. They were named "natural killers" because they do not require activation to kill cells that are missing "self" markers of MHC class 1. This makes NK cell so important because harmful cells that are missing MHC I markers cannot be detected and destroyed by other immune cells, such as T lymphocyte cells. In contrast to NKT cells (CD3+56+), NK cells do not express T-cell antigen receptors (TCR) or pan T marker CD3. They usually express the surface markers CD16 (FcγRIII) and CD56 I humans and NK1.1 or NK1.2 in mice.