Following a period of tight adherence, the leucocyte is seen to extend pseudopodia and move into the interstitium. Requirements for transmigration include, endothelial cell activation, expression of adhesion molecules, production of inflammatory mediators, cytoskeletal reorganization and alterations in membrane fluidity.
The critial molecular interations appear to be between integrin to ICAM-1 binding and integrin to VCAM-1 binding. Monoclonal antibodies against CD11/CD18 receptors on the leucocyte and ICAM-1 on the endothelium halt transmigration. L and E selectins do not play a role.
If a chemoattractant is present the leucocyte moves from the subendothelial space towards the chemoattractant.