Complement component C5a (C5a) is a protein fragment generated upon cleavage of complement component C5. It is a potent chemotactic factor for human peripheral blood neutrophils and monocytes and is believed to play an important role in a number of inflammatory conditions. The major functions of C5a are as follows: C5a is an anaphylatoxin, causing the release of histamine from mast cells; C5a is an effective leukocyte chemoattractant, promoting the accumulation of white blood cells, especially neutrophil granulocytes, at sites of complement activation; C5a activates white blood cells by increasing integrin avidity and upregulating the lipoxygenase pathway for arachidonic acid metabolism; C5a is a powerful inflammatory mediator and appears to be a key factor in the pathogenesis of many inflammatory diseases involving the complement system; and C5a modulates the balance between activating and inhibitory IgG Fc receptors on leukocytes, a mechanism that may enhance autoimmune responses under pathological conditions.