Second, the C5a/C5aR signalling upregulates the CD11b expression of neutrophils and promotes Mac-1 (CD11b/CD18) binding to Intercellular Adhesion Molecule-1 (ICAM-1) of vascular endothelial cells, which mediates trans-endothelial migration of neutrophils to the infection loci. Here, ICAM1 is linked to infection.