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Organs in the immune system include the bone marrow, spleen and thymus. The bone marrow is the place where blood cells are formed and white blood cells, or leukocytes, play an important part in the body's immune response against bacteria, viruses and other pathogens. Some of the blood cells, known as T lymphocytes, mature inside the thymus gland. The spleen contains T lymphocytes and B lymphocytes which respond to harmful substances, or antigens, in the blood. Some of the T cells destroy antigens, while some help activate other lymphocytes, and B cells are able to produce antibodies.
Two different tissue types are found in the spleen, known as white pulp and red pulp. Inside the red pulp, old red blood cells are removed from the circulation and broken down, and platelets are stored. Platelets are involved in blood clotting, and the spleen can release its extra stores in the case of a serious hemorrhage. B and T lymphocytes are found in the white pulp, and these cells have receptors which enable them to recognize and destroy specific antigens.
The spleen is one of the organs in the immune system that it is possible to do without, although after its removal, a patient may be at greater risk of infections. Another such organ is the thymus gland, which is found below the sternum, or breast bone. An adult can live without a thymus, although the gland is necessary in babies if they are to have a functioning immune system. Of all the organs in the immune system, the bone marrow is the one that is essential for life, because without it the body is unable to produce new blood cells.
Although the thymus contributes to immunity throughout life, and continues to produce T lymphocytes into old age, the gland shrinks after puberty. Its activity decreases, and the glandular tissue is replaced by areas of scarring and fat. The most important of the organs in the immune system, the bone marrow, also shrinks with age, with functional tissue being partly replaced by fat. Bone marrow produces what are called stem cells, which can develop into red or white blood cells, or platelets. The white cells, which are involved in disease protection, circulate in the blood, with the lymphocytes entering the lymphatic system.
Millions of new blood cells are produced daily in the bone marrow. New cells are constantly required, especially as some of the white blood cells have a lifespan of only a few days. If bone marrow is destroyed by disease, or an aggressive cancer treatment, a bone marrow transplant may be necessary.