KAR meaning in Medical ?

Killer Activation Receptors (KARs) are receptors expressed on the plasmatic membrane of Natural Killer cells (NK cells). KARs work together with inhibitory receptors (abbreviated as KIRs in the text), 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, the differences tend to be in the intracellular domains. KARs and KIRs can have tyrosine containing activatory or inhibitory motifs in the intracellular part of the receptor molecule (they are called ITAMs and ITIMs).

At first, it was thought that there were only one KAR and one KIR (two-receptor model). In the last decade, many different KARs and KIRs, such as NKp46 or NKG2D, have been discovered (opposing-signals model). NKG2D is activated by the cell-surface ligands MICA and ULBP2.

There is an unfortunate confusion about the KIR acronym. The KIR term has been started to be being used parallelly both for the Killer-cell immunoglobulin-like receptors (KIRs) and for the Killer Inhibitory Receptors. The Killer-cell immunoglobulin-like receptors involve both activatory and inhibitory receptors. Killer-cell inhibitory receptors involve both immunoglobulin-like receptors and C-type lectin-like receptors.