As blood circulates under pressure, its fluid component
(plasma) seeps through the thin wall of the capillaries into the surrounding
tissue. Much of this fluid, called interstitial fluid, returns to the blood
through the capillary membranes. The remainder of the interstitial fluid, now
called lymph, flows from the spaces in connective tissue into a network of tiny
open lymphatic capillaries and then into a series of progressively larger
collecting vessels called lymphatic vessels (Figure 2-16).
The largest lymphatic vessel, the thoracic duct, empties
into the left subclavian vein near the heart (see Figure 2-13). In this way,
the lymphatic system captures fluid lost from the blood and returns it to the
blood, thus ensuring steady-state levels of fluid within the circulatory
system. The heart does not pump the lymph through the lymphatic system; instead
the flow of lymph is achieved as the lymph vessels are squeezed by movements of
the body’s muscles. A series of one-way valves along the lymphatic vessels
ensures that lymph flows only in one direction.
When a foreign antigen gains entrance to the tissues, it is
picked up by the lymphatic system (which drains all the tissues of the body)
and is carried to various organized lymphoid tissues such as lymph nodes, which
trap the foreign antigen. As lymph passes from the tissues to lymphatic
vessels, it becomes progressively enriched in lymphocytes. Thus, the lymphatic
system also serves as a means of transporting lymphocytes and antigen from the
connective tissues to organized lymphoid tissues where the lymphocytes may
interact with the trapped antigen and undergo activation.
Source : Richard A. Goldsby, Thomas J. Kindt, And Barbara A. Osborne. 2000. KUBY IMMUNOLOGY 4th Edition. New York. W. H. FREEMAN AND COMPANY. Page 46.
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