The Blood and Lymphatic Systems
As mentioned already, many cells of the immune system, particularly lymphocytes, NK cells, monocytes, neutrophils, eosinophils, and basophils travel around the body in the blood and lymph. Both the blood vessels and lymphatic vessels are lined with a type of epithelial cell referred to as endothelium. As infectious agents can, collectively, infect any organ or tissue, this motility of the immune system is essential in order to protect the whole body. Leukocytes are carried through the blood circulation by the pumping action of the heart, and travel from the heart through the arteries to eventually reach the capillaries found throughout the tissues. The leukocytes can continue their journey in the veins, which contain internal valves to ensure the blood continues to flow in the correct direction, eventually leading back to the heart. Thus leukocytes can go around the body again and again via the blood circulation. The system of lymphatic vessels (Figure 6.10) is also distributed throughout the body and makes physical connections with the blood circulation in the thorax (the chest). Here a lymphatic vessel called the thoracic duct (also referred to as the left lymphatic duct) joins up with the left subclavian vein, while the right lymphatic duct joins to the right subclavian vein.
Small lymphatic capillaries collect interstitial fluid (the fluid that surrounds and bathes cells) and join up with each other to form the afferent lymphatic vessels. The various motile cells of the immune system, and any pathogens or fragments of pathogens that might be present, can also be carried with the interstitial fluid into the afferent lymphatics. The fluid is now referred to as lymph and moves through the lymphatic vessels because of the peristaltic activity of the vessels coupled with valves that ensure unidirectional flow. The afferent lymphatic vessels eventually enter organized lymphoid structures called lymph nodes. The lymph can subsequently leave the lymph nodes via the efferent lymphatic vessels, and will eventually mix with the blood using the connections we have described earlier.