<P> Leukocyte extravasation, less commonly called diapedesis, is the movement of leukocytes out of the circulatory system and towards the site of tissue damage or infection . This process forms part of the innate immune response, involving the recruitment of non-specific leukocytes . Monocytes also use this process in the absence of infection or tissue damage during their development into macrophages . </P> <P> Leukocyte extravasation occurs mainly in post-capillary venules, where haemodynamic shear forces are minimised . This process can be understood in several steps, outlined below as "chemoattraction", "rolling adhesion", "tight adhesion" and "(endothelial) transmigration". It has been demonstrated that leukocyte recruitment is halted whenever any of these steps is suppressed . </P>

When do white blood cells exit the vascular system