What are natural killer cells?

Natural killer cells, also known as NK cells, are large, granular, bone-marrow- as well as lymph node-derived lymphocytes. They represent the third largest lymphoid cell population in mammals, following B and T lymphocytes, and play a critical part in the innate immune system. NK cells contribute to the first line of defense against virus-infected or malignantly transformed cells. Unlike T cells or B cells, NK cells do not require sensitization for the expression of their activity, i.e. they have the ability to recognize and kill stressed cells in the absence of antibodies and major histocompatibility complex (MHC), allowing for a much faster immune reaction.