"140 which becomes swollen as a result. One of the proteins released during the inflammatory response also makes the area more sensitive to painful st imuli, so inflamed tissue is sore to touch. Sites of tissue injury are vulnerable to invasion by pathogens so the benefits of inflammation generally outweigh the discomfort it causes. Flooding the area with fluid dilutes any pathogens that are already pres ent, and the local concentration of immune system cells and defensive proteins enables an immune response to begin more quickly. Leukocytes: the cells of the immune system Once the barriers to infection have been breached and inflammation has begun, the ac tive agents of the immune system, the leukocytes [loo - koh - sites], get to work. Leukocytes are often described as ‘white blood cells’ to distinguish them from the red blood cells that transport oxygen around the body; however, calling them ‘blood’ cells is misleading because leukocytes roam throughout the body tissues and only spend part of their lives in the bloodstream. In fact, they spend more time in the lymphatic system (Figure 8), the network of fine tubules that collect tissue fluid from all over the body and return it to the bloodstream. Figure 9 . The human lymphatic system"