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The thymus is a small, irregular-shaped gland in the top part of the chest, just under the breastbone and between the lungs. It is located in an area of the body called the mediastinum. The thymus is part of both the lymphatic system and the endocrine system.
Structure of the thymus
The thymus is divided into 2 main parts – a right lobe and a left lobe. Each lobe is divided into smaller sections called lobules that give the thymus its bumpy appearance. Each lobule is made up of a centre part (called the medulla) and an outer layer (called the cortex). A thin covering (capsule) surrounds and protects the thymus.
The thymus changes in size as you get older. It is large in newborns and toddlers. It is biggest during puberty then slowly begins to shrink as adulthood approaches.
The thymus is most active during childhood and youth. By late adulthood, most of the thymus is made up of fat tissue.
What the thymus does
The thymus makes T cells (T lymphocytes) that travel throughout the body to help fight infection, disease and foreign substances. The thymus also makes hormones to help T cells develop and keep the immune system working properly.
Lymphocytes travel from the bone marrow to the thymus, where they mature into T cells. Once T cells mature, they are able to leave the thymus and enter the blood so they can help the immune system. T cells also travel to lymph nodes and the spleen where they continue to mature.
A specialized organ or group of cells that produces or releases substances (such as hormones, saliva, digestive juices, sweat, tears or milk) to perform different functions in the body.
A specialized cell that makes up the epithelium (a layer of cells that makes up the surface of the skin, and lines cavities, glands and passages in the body). Some epithelial cells make mucus, hormones or other secretions.
The 4 types of epithelial cells are squamous cells, columnar cells, cuboidal cells and transitional cells.
A type of white blood cell that fights viruses, bacteria, foreign substances or abnormal cells (including cancer cells).
The 3 types of lymphocytes are B cells, T cells and natural killer (NK) cells.
A substance that regulates specific body functions, such as metabolism, growth and reproduction.
Natural hormones are produced by glands. Artificial or synthetic hormones can be made in the lab.
The complex group of cells and organs that defend the body against infection, disease and foreign substances.
The soft, spongy tissue inside most bones.
There are 2 main types of bone marrow. Red bone marrow is where blast cells (immature blood cells) develop into red blood cells, white blood cells and platelets. Yellow bone marrow stores fatty tissue.
A small, bean-shaped mass of lymphatic tissue along lymph vessels (tubes through which lymph fluid travels in the body). Lymph nodes store lymphocytes (a type of white blood cell that fights germs, foreign substances or cancer cells) and filters bacteria and foreign substances (including cancer cells) from lymph fluid.
The organ on the upper-left side of the abdomen near the stomach that makes lymphocytes (a type of white blood cell that fights germs, foreign substances or cancer cells), stores blood cells, filters the blood and destroys old blood cells.