Signal regulatory protein alpha (SIRP alpha, CD172a), also known as SHPS-1 and formerly MyD-1, is a ~90 kDa type I transmembrane glycoprotein in the SIRP/SHPS (CD172) family of the immunoglobulin superfamily. SIRPs are paired receptors with similar extracellular domains but distinct C-termini and functions. The human SIRP alpha, consisting of 503 amino acids, includes a 342 amino acid extracellular domain with one V-type and two C1 type Ig domains, and three potential N-glycosylation sites. It features a 110 amino acid cytoplasmic sequence with ITIM motifs that recruit tyrosine phosphatases SHP-1 and SHP-2 upon phosphorylation. SIRP alpha exhibits over 40 polymorphisms, including the notable BIT (also known as SIRP alpha 2 or PTPNS). Engagement of SIRP alpha generally elicits a negative regulatory signal. It recognizes CD47 on aged erythrocytes, platelets, or xenogenic cells, promoting their clearance from circulation. In the lung, SIRP alpha's recognition of surfactants SP-A and SP-D can inhibit alveolar macrophage cytokine production. Additionally, the CD47-SIRP alpha interaction facilitates macrophage fusion during osteoclastogenesis.