White blood cells migrate through endothelial walls of capillaries and venules and enter tissue spaces by this process. What is it called?

• A. Diapedesis
• B. Phagocytosis
• C. Chemotaxis
• D. Hematopoiesis

Answer: (A)

Leukocyte diapedesis is the process by which white blood cells exit the bloodstream by crossing the endothelial lining of capillaries and postcapillary venules to reach tissue spaces. This transmigration is essential during inflammation, allowing immune cells to reach sites of infection or injury. The cells first roll along the endothelium, adhere firmly via integrins, and then squeeze through endothelial junctions (paracellular) or, less commonly, through the cells themselves (transcellular). Molecules like PECAM-1 help at the junctions, and chemokines from affected tissue activate the leukocytes to move toward the target site (chemotaxis). This is distinct from phagocytosis (ingesting particles after arrival) and from hematopoiesis (production of blood cells) and from chemotaxis alone (directed movement without crossing the vessel wall).