L-selectin is a type of protein found on the surface of certain white blood cells, specifically leukocytes. It plays a crucial role in the immune system by helping these cells adhere to blood vessel walls, which is essential for their movement to sites of inflammation or infection. When L-selectin binds to specific carbohydrates on the endothelial cells lining blood vessels, it initiates a process called rolling. This allows leukocytes to slow down and eventually exit the bloodstream, enabling them to reach areas where they are needed to fight off pathogens or facilitate healing.