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How do white blood cells leave blood vessels and enter injured tissue?

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Pathways of complement activation — inflammation The main function of complement proteins is to aid in the destruction of pathogens by piercing their outer membranes (cell lysis) or by making them more attractive to phagocytic cells such as macrophages (a process known as opsonization). Some complement components also promote inflammation by stimulating cells to release histamine and by attracting phagocytic cells to the site of infection.

White blood cells, or leukocytes, exit the bloodstream to reach injured tissue through a process facilitated by increased blood vessel permeability. During the inflammatory response, blood vessels dilate and their walls become more permeable, allowing these cells to migrate from the circulation into the surrounding tissue.12

Once they have exited the vessels, the movement of these cells toward the site of injury is guided by chemotaxis. In this process, chemical substances known as chemotactic factors diffuse from the damaged area, creating a concentration gradient that the white blood cells follow.3

Neutrophils are typically the first to arrive, often within an hour of the injury, to begin ingesting bacteria and cellular debris. They are followed later by monocytes, which mature into macrophages to continue the cleanup and support tissue repair.231