The monoclonal antibody BOB93 binds to the surface of apoptotic neutrophils. Neutrophils have beenimplicated in the pathogenenesis of a variety of inflammatory diseases including the adult respiratorydistress syndrome, idiopathic pulmonary fibrosis, ulcerative colitis and rheumatoid arthritis. Although theneutrophil is a vital component of the body’s defense against infectious agents, uncontrolled release ofits toxic substances may inflict “friendly fire” damage on surrounding tissue and propagate theinflammatory response, leading to scarring and tissue destruction. Apoptosis leads to recognition andsafe disposal of dying cells by phagocytosis. Binding of the monoclonal antibody BOB93 to apoptoticneutrophils is dependent on the presence of sialoglycoprotein fetuin, a constituent of bovine serum.Fetuin is the antigen for BOB93, and BOB93 and fetuin form a complex in solution that is necessary andsufficient for binding to apoptotic neutrophils. The antigen recognised by BOB93 may act as a“molecular bridge” between the surface of the apoptotic neutrophil and the phagocyte to modulateapoptotic neutrophil clearance at inflammatory sites. Human macrophage phagocytosis of apoptoticneutrophils was augmented in vitro by addition of the antigen recognised by BOB93, which does notbind to other apoptotic leukocytes.