Natural killer (NK) cells play a critical role in the innate immune system and represent a first line of defense against pathogens and tumor cells. Involved in immune surveillance, NK cells patrol the body looking for infected or cancerous cells. While other immune cells recognize the presence of antibodies on the MHC receptor of infected cells before a response is initiated, NK cells have the ability to respond to stressed cells in the absence of the MHC receptor, allowing a much faster immune response. The lack or downregulation of MHC receptors on the surface of target cells triggers NK cell cytotoxicity, allowing NK cells to destroy tumor and virus-infected cells by lysis. At the same time, NK cells stimulate and guide other cells, including lymphocytes (T and B cells) of the adaptive immune system, to the area of infection to help increase the immune response and generate memory and effector cells that aid in forming a strong response if the infection should arise again.
