Complement component 5a (C5a) is a pro-inflammatory protein fragment anaphylatoxin cleaved from complement component C5. It is a strong chemoattractant for leukocytes in stimulating an inflammatory response, and is recruited with the complement system network during an immune response to tissue damage or foreign objects. It is widely involved in many forms of inflammatory diseases, including asthma, ischemia-reperfusion injury, acute lung injury, and sepsis.