In various forms of vascular pathology, MPO is deposited in the vasculature after its release from neutrophils, binding to negatively charged glycosaminoglycans on the endothelial surface. Due to its strong positive charge, deposited MPO can attract leukocytes to the endothelial surface. However, this does not happen wherever MPO is deposited in the vasculature, but only in sites where leukocyte rolling and adhesion normally occur, such as postcapillary venules. This indicates that the attractive force of MPO is insufficient to initiate adhesion, but can augment adhesion mediated by conventional adhesion molecule pathways. (Professional illustration by Alice Chen.)