This was originally posted as a response to a question on NK cells.
NK and Tc Cells
NK cells, or "large granular lymphocytes" as they are also known, and cytotoxic T (Tc) cells have related function. One textbook refers to them as the Police and Secret Police of the anti-viral world.
Tc cells identify viral infection by inspecting the target cells credentials. Each cell has an "antigen presenting complex" (also known as the major histocompatibility complex, or MHC) which samples some of the proteins in the cell and presents them for inspection by the Tc cells. If the Tc cell detects "foreign" protein (such as when a virus has hijacked the cell), it tells the infected cell to commit suicide (apoptosis), thus confounding the virus's plans for world domination.
However, some viruses (HIV and adenovirus, as two examples) play dirty and instead of allowing this to happen, they prevent the MHC from existing at all. This means the Tc cells can't inspect them, and that's where NK cells come in.
NK cells kill cells which DON'T present their credentials - their cytotoxic role is inhibited when a protein called "Killer Inhibitory Receptors" interact with the MHC.
However, NK cells can use immunoglobulins to detect infected cells: they achieve this by having an adaptor on their surface which can attach to the tail of IgG, so they work with antibodies to identify and kill virus-infected cells.
Tc and NK cells both have a number of techniques for killing cells. They typically use apoptosis : they either instruct the cell to commit suicide - apoptosis - or (if that fails) they release a protein which punches holes in the cell's outer membrane - the protein is called "perforin". Not only does this mean the cell starts leaking (which results in death), but the cells are then injected (through the perforation) with enzymes which override the cell and initiate the apoptosis sequence.
