The Innate Immune System

Natural killer, or NK cells, develop from lymphoid progenitors, but play an important role in the innate immune system. Their action is similar to that of cytotoxic T lymphocytes in that they target viral-infected host cells, or host cells that have become damaged or malignant. They differ from T lymphocytes, however, in that they produce a rapid response, they lack immunological memory and they do not require priming by an antigen in order to initiate a response. Natural killer cells are primarily involved in recognising and targeting host cells that have a reduced or absent MHC I signal. This is useful, as sometimes viruses and malignant cells can try to evade the immune system by reducing the MHC I signal expressed on the surface of host cell membranes. In addition, harmful cells that are missing their MHC I markers cannot be detected and destroyed by any other immune cell.