Selectin is a type of protein that plays a crucial role in the immune system. It helps white blood cells adhere to the walls of blood vessels, allowing them to exit the bloodstream and reach sites of inflammation or injury. This process is essential for the body's response to infections and healing. There are three main types of selectins: E-selectin, P-selectin, and L-selectin. Each type is found on different cells and has specific functions. For example, E-selectin is primarily expressed on endothelial cells lining blood vessels, while P-selectin is found on platelets and activated endothelial cells.