Signal regulatory protein a(SIRPa) is a transmembrane peotein with an extracellular region comprising three Ig-like domains and a cytoplasmic region containing immunoreceptor tyrosine-based inhibition motifs which mediate binding of the protein tyrosine phosphatases SHP1 and SHP2. SIRPa is especially abundant in myeloid cells such as macrophages and dendritic cells(DC),whereas it is expressed at very low levels in T,B,and NK,and NK T cells.