Final answer:
Leukocytes express integrins to adhere to endothelial cells before moving into the vessel wall at the site of infection, in a response guided by chemical attractants.
Step-by-step explanation:
At the site of infection, leukocytes are stimulated to express integrins, which are proteins that enable the leukocytes to attach to the endothelial cells before moving into the vessel wall. This process is crucial for the leukocytes' ability to exit blood vessels and migrate toward the site of injury or infection.
Leukocytes in the blood respond to chemical attractants released by pathogens and chemical signals from nearby injured cells. When they encounter these signals, leukocytes increase their expression of integrins, which bind to molecules on the endothelial cells, facilitating a process known as adhesion. Following adhesion, the leukocytes can then pass between the endothelial cells and move into the tissue in response to chemotactic signals, a process called extravasation.
Chemotactic factors, such as chemokines, also play a role in guiding leukocytes to the site of infection or inflammation, however, they are not directly involved in the initial attachment to the endothelial cells.