The morphological behaviour of leucocytes* leaving the blood has been studied for more than a hundred years and can be described in some detail, but the mechanisms involved are for the most part still unclear. Following an inflammatory stimulus, dy- namic changes occur in small blood vessels near the inflamed site and are followed by margination of leucocytes and by their adherence to the endovascular surface of endo- out of the vessel in the gaps between endothelial thelial cells. The leucocytes migrate cells and move towards the noxious stimulus, directed presumably by chemotactic gra-...