Although the biological effects and functional properties of the potenl chemolactic factor C5a have been known since the 1960s, the C5a receptor (C5aR) was not demonstrated on human leukocytes or characterized until the late 1970s. Only in 1991 was the gene structure and derived protein sequence for human C5aR first reported. The C5aR, also known as CD88, is a member of the rhodopsin superfamily of receptors with wide cellular and tissue distribution in both humans and animals. As the receptor for a major humoral chemotactic factor and potent immunomodulatory factor, it is a key component of inflammatory and immunoregulatory processes. The nature and variety of cellular functions associated with the engagement of C5aR by its natural ligand. and the physiologic consequences of the mediated cellular responses, place C5aR in the same category functionally as chemokine and cytokine receptors.