High Quality Content by WIKIPEDIA articles! Killer Activation Receptors (KARs) are receptors expressed on the plasmatic membrane of Natural Killer cells (NK cells). KARs work with Killer Inhibitory Receptors (KIRs), which inactivate them in order to regulate the NK cells functions on hosted or transformed cells. These two kinds of specific receptors have some morphological features in common, such as being transmembrane proteins. The similarities are specially found in the extracellular domains and, therefore, the differences tend to be in the intracellular domains. KARs and KIRs can only do their function in presence of immunoreceptors that contain tyrosine and have an activator or inhibitor function (they are called ITAMs and ITIMs).