May be a lipoprotein-borne regulator of either the coagulation or the complement cascades. Forms high molecular weight disulfide-linked complexes. Plasma. Found on very low-density lipoprotein (VLDL), on chylomicrons, and in the D > 1.21 g/ml fraction of pig plasma. Found in liver, spleen, lung, bone marrow and lymph node. Terminally differentiated macrophages.