<prompt> The following features is common to both cytotoxic T cell and NK cellSynthesis antibodyRequire antibiodies to be present for actionEffective against virus infected cellsRecognise antigen in association with HLA class2 markers  <response>: Ref Harrison 17e p2024-2028,9e p192 Natural killer cells, or NK cells, are a type of cytotoxiclymphocyte critical to the innate immune system. The role NK cells play is analogous to that of cytotoxic T cells in the veebrate adaptive immune response. NK cells provide rapid responses to virus-infected cells, acting at around 3 days after infection, and respond to tumor formation. Typically, immune cells detect the major histocompatibility complex MHC presented on infected cell surfaces, triggering cytokine release, causing lysis or apoptosis. NK cells are unique, however, as they have the ability to recognize stressed cells in the absence of antibodiesand MHC, allowing for a much faster immune reaction. They were named natural killers because of the initial notion that they do not require activation to kill cells that are missing self markers of MHC class 1. This role is especially impoant because harmful cells that are missing MHC I markers cannot be detected and destroyed by other immune cells, such as T lymphocyte cells. 