C5b is important in late events of the complement cascade, an orderly series of reactions which coordinates several basic defense mechanisms, including formation of the membrane attack complex (MAC), one of the most basic weapons of the innate immune system, formed as an automatic response to intrusions from foreign particles and microbial invaders.

C5a, the other cleavage product of C5, acts as a highly inflammatory peptide, encouraging complement activation, formation of the MAC, attraction of innate immune cells, and histamine release involved in allergic responses.

NMR spectroscopy proved that the molecule is composed of four helices and connected by peptide loops with three disulphide bonds between helix IV and II, III.

C5aR1 is a member of the G-protein-coupled receptor superfamily of proteins, predicted to have seven transmembrane helical domains of largely hydrophobic amino acid residues, forming three intra- and three extra-cellular loops, with an extracellular N-terminus and an intracellular C-terminus.

The latter interaction leads to receptor activation, and the transduction of the ligand binding signal across the cell plasma membrane to the cytoplasmic G protein Gi type GNAI2.

C5a, acting via C5aR1, is shown to differentially modulate lipopolysaccharides-induced inflammatory responses in primary human monocytes versus macrophages,[7] yet this is not due to C5aR1 upregulation.