Lymphocyte interaction with endothelium. In the postcapillary venules, selectin-sialomucin interactions (or interactions mediated by integrin α4β1 or α4β7) mediate “rolling” of lymphocytes on the endothelium. Chemokines, presented by heparan sulfate proteoglycans (HSPGs) expressed on the endothelium, bind to chemokine receptors, which are G protein–coupled receptors, leading to increased affinity/avidity integrins on the surface of lymphocytes. Interaction of these integrins with their ligands results in stable adhesion of lymphocytes to endothelium and in diapedesis, involving engagement with junction adhesion molecules (JAMs) and PECAM-1 (CD31).