Immunoglobulin-like transcript 5 (ILT5), also known as leukocyte immunoglobulin-like receptor subfamily B (LILRB3), leukocyte immunoglobulin-like receptor 3 (LI R3) and CD85a, is an Ig-like recept or family member that is involved in immune regulation. ILT5 belongs to subfamily B whose members have cytoplasmic tails that contain immunoreceptor tyrosine-based inhibitory motifs (ITIMs) that inhibit signaling events via phosphatase SHP-1. The counterpart to subfamily B is subfamily A, whose members qualify as activating receptors that lack ITIMs, but signal through association with FcRg . ILTs share structural homology and chromosomal localization with the KIR family of receptors (1, 2). Mature ILT5 is a highly polymorphic 85-95kD glycoprotein that consists of a 420aa extracellular domain with four Ig-like domains, a 21aa transmembrane segment, and a 167aa cytoplasmic domain with three ITIMs (3, 4). Alternate splicing generates an isoform with a 17aa insertion in the juxtamembrane ECD. In mouse and rat , the LILRB3 gene encodes the PIR