C3 is the third component of the complement system and plays a central role in the complement cascade. The three pathways of compliment activation come together- at this key molecule. Cleavage of the pivotal complement component C3 results in the generation of a set of effector molecules which have diverse biological functions. The cleavage product C3b promotes complement activation and the subsequent formation of the membrane attack complex. C3a, the small (ca. 10 kDa) cleavage product possesses anaphylatoxic as well as immunoregulatory- properties. It exerts its function through a specific receptor (C3aR), which belongs to the G-protein coupled receptor family. Expression of C3aR- has been reported in many cell types including myeloid and non-myeloid cells. C3a is a protein of 74 amino acids. C3a itself is very short-lived and in serum is cleaved rapidly into the more stable C3a-desArg (also called acylation stimulating protein, ASP). Expression of C3aR on haematopoietic stem/progenitor cells has been shown to promote the trafficking/homing of these cells to the BM. Engagement of C3aR on DCs and T cells has been shown to up-regulate these cell functions.