What is leukocyte movement through vessel walls called?

Diapedesis is the specific term for the process by which leukocytes (white blood cells) squeeze through the walls of capillaries and venules to migrate from the bloodstream into surrounding tissue. This occurs during inflammation when chemical signals attract immune cells to a site of injury or infection. The process involves leukocytes first marginating and rolling along the endothelium, then firmly adhering via cell adhesion molecules, and finally transmigrating between endothelial cells. Chemotaxis is a related but distinct concept — it describes the directional movement of leukocytes toward the site of inflammation in response to chemical gradients (chemokines, complement fragments, bacterial products), but it refers to movement through tissue, not through vessel walls. Phagocytosis is the process by which neutrophils and macrophages engulf and destroy pathogens or debris once they have already arrived at the tissue site. Agglutination refers to the clumping together of particles, cells, or bacteria, often in the context of antibody-antigen reactions, and has no specific role in leukocyte migration through vessel walls. Diapedesis is therefore the only term that precisely describes transmigration through the vessel wall itself.