C3a, along with C4a and C5a, is a 77 amino acid anaphylotoxin generated by proteolytic cleavage during activation of the complement pathway. The anaphylotoxins strongly promote inflammation by recruiting leukocytes, particularly basophils, eosinophils, neutrophils and monocyte. The proinflammatory properties of C3a are mediated by interaction between the peptide and a 7-TM G-protein coupled receptor, C3aR. Genetic and pharmacological inhibition of C3a/C3aR interaction indicates an important role for C3aR in allergic asthma. C3a/C3aR also enhances the effect of SDF-1 in promoting retention of haematopoietic stem/progenitor cells within the bone marrow.