Clone S5/1 recognizes the human CD88 antigen, a 43 kDa G type protein–coupled receptor with seven membrane-spanning regions which is also known as C5a anaphylatoxin chemotactic receptor 1 (C5AR). CD88 expression is originally described on neutrophils and monocyte/macrophages, but it is also found on hepatocytes, epithelial cells, endothelial cells, tissue mast cells, and human fetal astrocytes. Constitutive CD88 expression was also detected on astrocytes, microglia, and fibroblast-like cells but not by oligodendrocytes. CD88 is a receptor for the chemotactic and inflammatory peptide anaphylatoxin C5a. It stimulates chemotaxis, granule enzyme release, and superoxide anion production. Sulfation plays a critical role in the association of CD88 with C5a, but no significant role in the ability of CD88 to transduce a signal and mobilize calcium in response to a small peptide agonist.