The minor fragment formed when C5 convertase cleaves C5 into C5a and complement c5b. C5a is a 74-amino-acid glycopeptide with a carboxy-terminal arginine that is crucial for its spasmogenic activity. Of all the complement-derived anaphylatoxins, C5a is the most potent in mediating immediate hypersensitivity (hypersensitivity, immediate), smooth muscle contraction; histamine release; and migration of leukocytes to site of inflammation.