Final answer:
The vascular system, specifically the lymphatic vessels, enables immune system cells to respond by transporting them to the sites of infection or injury.
Step-by-step explanation:
The vascular system, specifically the network of lymphatic vessels, enables the cells of the immune system to respond. The lymphatic system functions as a transportation system for the cells of the immune system, allowing them to circulate throughout the body and reach the sites of infection or injury. The lymphatic vessels carry lymphocytes, which are a type of white blood cell involved in the immune response, to the lymph nodes where they can encounter and respond to pathogens.