Presumably, the most important function is the induction of an anaphylactoid reaction. The small activation products C3a, C4a and particularly C5a are potent anaphylatoxins, capable of inducing the migration of phagocytes (29) , smooth muscle relaxation, degranulation of mast cells and basophile granulocytes and therefore unleashing vasoactive substances such as histamine, prostaglandins, kinins and serotonin. All can cause vasodilation and capillary leakage (30) and induce the migrated cells to release eicosanoids, oxygen radicals and lysosomal enzymes, which cause damage to the pathogens (31) (32) (33) . It is noteworthy, that complement acts far beyond "inflammation," as indicated by its close interaction with the coagulation cascade (34) (35) (36) and its involvement in the regulation of apoptosis (37) (38) (39) (40) and cellular growth (41) . The anaphylactoid functions are mediated by the interaction of C3a and C5a with their corresponding seventransmembrane-spanning receptors C3aR and C5aR (CD88), respectively (see Figure 2 ). The role of the second C5a receptor named C5L2 is not fully understood and is still controversially discussed. However, there is increasing evidence that C5L2 represents a functional receptor acting as a negative regulator of the inflammatory response. For example, it was shown that inflammation in C5L2 knock-out mice was amplified (42) and that blockage of C5L2 increased serum interleukin (IL)-6 (43).