The leukocyte on the left is shown circulating in the lumen of a post-capillary venule (A). The wall of these vessels has potential or actual spaces between adjacent endothelial cells. Such spaces are the sites of migration of motile cells from the lumen of the vessels to the perivascular spaces.

In response to the chemotactic, or attractive, substances produced by bacteria, the neutrophil marginates, that is, it establishes contact with the wall of the vessel (B). Then, with its ameboid motion, the cell crosses the wall of the vessel (C). This process, called diapedesis, is a response to chemotactic stimulation. Next, owing to its chemotactism, the neutrophil migrates in the tissue toward the bacteria (D). When the neutrophil establishes contact with bacteria, it engulfs or phagocytizes and lyses the pathogens (E). The accumulation of dead neutrophils and macrophages or other cellular debris at the site of infection produces pus. This pus is eventually eliminated by the phagocytic action of additional macrophages derived from the blood monocytes.