Endocrine System

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The Thymus Gland

5.5     The thymus is a flat, pinkish-grey organ that plays an important role in the immune system of the body. It is located high in the upper chest cavity behind the breastbone and extends into the lower neck below the thyroid gland. At birth, the thymus is about 15 grams. By the age of 12, it grows to about 30 grams, but by adulthood, the organ shrinks so much that it may be hard to distinguish it from the fatty tissue that surrounds it. The thymus helps in the development of white blood cells in the bone marrow.

 White Blood Cells

5.6     As integral component of the immune system, the white blood cells or leukocytes, defend the body against infection and disease. They begin their lives as immature cells in the bone marrow. As they mature, leukocytes become specialised for specific functions. The leukocytes that are important in the immune response are the macrophages, neutrophils and several types of lymphocytes. The macrophages can surround and digest harmful bacteria and other invaders. Some can move from the bloodstream to the fluid that surrounds the body cells to reach the site of infection. The lymphocytes, like trained “soldiers” are involved in the production of antibodies (defence proteins) in recognising and destroying specific types of cells, and in remembering a specific pathogen (a disease causing microorganism) in case it is encountered again.

5.7   Some of the immature lymphocytes leave the bone marrow and go to the thymus gland. There they undergo a change and become one of the different types of T-cells (for thymus) of the immune system to inhabit the blood, lymph nodes and spleen. This change takes place just before birth and during the first months of life. The thymus also functions as an endocrine gland. Its hormone, thymosin, is important to the maturation of T-cells and is thought to influence them after they leave the thyroid.

Immune System Soldiers

5.8     Like any modern army, the immune system has specially trained “soldiers” equipped to handle specific attacks. Macrophages literally surround and engulf the enemy – harmful microorganisms; T-cells, sometimes, called “natural killers”, grapple with invaders including cancer cells, in hand-to-hand combat; T-cells produce antibodies specifically designed to seek out and destroy targeted germs; neutrophils serve as the foot soldiers for this internal army. Lightly armed, the neutrophils are often among the first to enter fray enmass sacrificing in large numbers to keep the body healthy. There are also helper T-cells to assist in “combat control”, and suppressor T-cells to help calm things down once the battle has been won.