The Immune System
The red blood cells are not one of the body's natural defenses against foreign invaders.
The Lymphatic System
A key component of the immune system is the lymphatic system. The lymphatic system can be thought of as a secondary circulatory system. The lymph vessels contain a clear, colorless fluid called lymph, which is derived from a network of capillaries which collect this clear fluid as it filters through the capillaries of the blood. The lymphatic system provides our immune defenses, filters foreign substances and cell debris from the blood and destroys them; and produces a type of white blood cells known as lymphocytes, which circulate in the blood and lymph vessels.
Lymph passes from tiny capillaries to lymph vessels and flows through lymph nodes that are located along the course of these vessels. Cells of the lymph nodes phagocytize, or ingest, such impurities as bacteria, old red blood cells, and toxic and cellular waste. Finally, the lymph flows into either the thoracic duct, a large vessel that runs parallel to the spinal column, or into the right lymphatic duct, both of which transport the lymph back into veins of the shoulder areas where is reenters the general circulation. All lymph vessels contain one-way valves, like the veins, to prevent backflow.
In an infection, the lymph nodes occasionally become enlarged with lymph and white blood cells and become palpable (can be felt by an examiner). These can be felt most easily at the neck, in infections of the neck and head; in the axillary region (the armpit), in infections of the breast or arm; and in the inguinal region (the groin), in infections of the pelvis or lower extremities.
Unlike the blood, which is forced through the arteries by the pumping action of the heart, the flow of lymph is not aided by a pump. The lymph moves by way of the one-way valves, aided by the compression of lymph vessels when the muscles of the body contract. Therefore, exercise and activity is very important for the proper circulation of the lymphatic fluid.
The tissues of the lymphatic system include the spleen, thymus, bone marrow, and aggregates of lymphatic tissue located in the tonsils and intestines (including the appendix). The spleen, thymus, and bone marrow manufacture lymphocytes, which are the major cell type of the system. The spleen is also involved in the destruction of old cells and other substances by phagocytosis and plays a role in immune responses. The thymus is considered the central organ that controls lymphocyte production and antibody formation.
Lymphatic tissues of the intestines are known as lacteals. They absorb digested fat (lipids), which are transported by the veins to the liver where they are processed for use by the body. Lymph fluid from lacteals is milky white because of the fat globules that are present.
The most common cell of the lymphatic system is the lymphocyte. Lymphocytes can be classified as T cells (thymus-derived) or B cells (bone-marrow-derived). Other cells of the lymphatic system include histiocytes, which are responsible for structural support of lymph tissue and for phagocytosis; monocytes, which also function to ingest foreign substances; and plasma cells, which synthesize and release antibodies. Monocytes are believed to originate from lymphocytes.
What is the most common cell type of the lymphatic system?