Signal Regulatory Protein Delta (SIRPd)

Signal regulatory proteins (SIRPs) constitute a family of transmembrane glycoproteins with extracellular Ig-like domains. Several SIRP family members have thus far been identified on myeloid and other cells in man, mouse, rat, and cattle. SIRPs as a diverse multigene family of immune receptors, which includes inhibitory SIRPa, activating SIRPb, nonsignaling SIRPg, and soluble SIRPd members.
For each species, there appears to be a single inhibitory SIRPa member that, upon interaction with the "self" ligand CD47, controls "homeostatic" innate immune effector functions, such as host cell phagocytosis. The activating SIRPb proteins show considerable variability in structure and number across species and do not bind CD47. Thus the SIRP family is a rapidly evolving gene family with important roles in immune regulation.