C5aR1, also known as C5a ligand and CD88, is a 7TM protein expressed on myeloid, endothelial, epithelial, and smooth muscle cells. C5aR binds the activated complement anaphylatoxin C5a. In established allergic environments, this triggers neutrophil and eosinophil chemotaxis and the release of proinflammatory mediators. In contrast, C5aR1/C5a interactions are protective during allergen sensitization. Human C5aR1 shares 66% amino acid sequence identity with mouse and rat C5aR.