Answer
Lymph is altered by several processes during its passage through lymph nodes. First there is the filtering process: macrophages and reticular fibers remove debris from the lymph. In addition, macrophages phagocytize microbes of various kinds. Also, in the lymph nodes, naive lymphocytes come into contact wit antigens being carried by the lymph. The result is that the lymphocytes are activated and readied to defend the body, immunologically. These processes take place slowly, and are therefore, facilitated by the slow flow-- near stasis-- of the passage of lymph through the lymph nodes.
Work Step by Step
One function of the lymphatic system is to return fluids to the blood. This does not require passing through lymph nodes. Lymph passes through lymph nodes to permit lymph nodes to process the lymph in several ways. First, the lymph is filtered. Second, macrophages phagocytize debris and microbes. In addition antigens in the lymph activate lymphocytes in the lymph node.. Thus the lymph that leaves a lymph is different from the lymph that entered from its afferent vessels. The so- called cleansing of lymph is a slow process; in fact, a given volume of lymph has to pass through several lymph nodes before it is "cleaned." Since the "cleansing" of lymph is a slow process , the slowing or stagnation of the lymph flow facilitates the cleansing, and activation processes.
Because there are fewer lymph vessels that enter a lymph node than the number that leave it, this means that lymph flow slows in the medullary sinuses.. Since lymphatics have one way valves, the lymph cannot flow backward when passage slows or stagnates.
