C1qR1 is also known as C1qRp, collectin receptor, and AA4 antigen. C1qR1 mediates enhanced phagocytosis by monocytes and macrophages upon interaction with soluble defense collagens including C1q, MBL, and SP-A. It is a type I transmembrane glycoprotein expressed on endothelial cells, hematopoietic progenitor cells, platelets, and cells of myeloid origin. C1qR1 has also been identified as a hematopoietic stem cell marker.