The Major Lymphatic Organs Help
Introduction to The Major Lymphatic Organs
The lymphatic system consists of a collection of lymphatic organs, as well as lymphatic vessels.
The Lymph Nodes
The most widespread lymphatic organs are the lymph nodes ( NOADS ). The lymph nodes are a group of small, bean-like organs scattered in clusters in various parts of the body (Figure 17.3). Afferent ( AF -fer- ent ) lymphatic vessels “carry” (fer) dirty lymph “towards” (af-) the lymph nodes. Efferent ( EE -fer- ent ) lymphatic vessels carry clean lymph “away from” (ef-) them.
The Thymus Gland
A most unusual lymphatic organ is the thymus ( THIGH -mus) gland. The word, thymus, comes from the Ancient Greek for “warty outgrowth,” reflecting the bumpy appearance of this endocrine gland. The thymus is a thin, flat gland lying just deep to the sternum ( STER -num) or “breastplate.” This gland consists of two bumpy-looking lobes. The thymus gland secretes the hormone, thymosin (thigh- MOH -sin), which stimulates the activity of the lymphocytes and other parts of the body’s immune system. It also produces thymic ( THIGH -mik) lymphocytes (introduced as T-cells or T-lymphocytes, earlier).
The thymus gland is most prominent in young humans and other mammals. (In lambs, it is called the “throat sweetbread,” because it is often eaten as sweet-tasting meat.) It reaches its maximum size at puberty, then progressively decreases in size. In most adults, the thymus is completely gone, having been replaced by fatty connective tissue. The thymus is thought to play an important role in the development of immune competence in youngsters – a growing ability to ward off various diseases. The disappearance of the thymus in adults may be related to the gradual decline of immune competence seen in older persons, thereby making them more susceptible to cancer and pneumonia.
Red Bone Marrow and Spleen
Red Bone Marrow
The red bone marrow found within spongy bone (Chapter 13) is a third important type of lymphatic organ. Besides its role in hematopoiesis (blood cell formation), the red bone marrow is a source of B-lymphocytes. As you may recall, the B-lymphocytes differentiate into plasma cells when they are chemically signaled by the T-lymphocytes that a foreign invader (antigen) is present. And these plasma cells, in turn, produce antibodies.
In humans, the spleen is a dark red organ attached to the left side of the stomach. It looks somewhat like a thick, crescent-shaped-roll (croissant), and it rather feels like one, being soft and spongy to the touch. Besides hematopoiesis, the spleen (like the red bone marrow) is involved in the recycling and destruction of old, beaten-up erythrocytes. It also stores blood and contains a lot of lymphatic tissue. The spleen is rich in lymphocytes, plasma cells, and wandering macrophages (mostly derived from monocytes). It is therefore an important helper in phagocytosis of foreign invaders, such as bacteria, as well as devouring fragments of broken erythrocytes.