Most C3b binds to antigens on the microbial surface. Some C3b combines with C2a and C4b to form the third enzyme of the complement pathway that is able to split C5 into C5a and C5b. C5a stimulates mast cells to release histamine for inflammation and diapedesis. It also functions as a chemoattractant for phagocytes. The phagocytes are then able to bind to the C3b attached to the surface of the microorganism allowing for opsonization (enhanced attachment).