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Heart and Respiratory System
The thymus is positioned in the upper chest cavity above the heart. Images


The thymus gland is the main organ of the lymphatic system. Located in the upper chest region, the primary function of this gland is to promote the development of specific cells of the immune system called T lymphocytes. T lymphocytes or T-cells are white blood cells that protect against foreign organisms (bacteria and viruses) that have managed to infect body cells. They also protect the body from itself by controlling cancerous cells. From infancy to adolescence, the thymus is relatively large in size. After puberty, the thymus begins to decrease in size and continues to shrink with age.

Thymus Anatomy

The thymus is a two-lobed structure that is positioned in the upper chest cavity. It partially extends into the neck region. The thymus is situated above the pericardium of the heart, in front of the aorta, between the lungs, below the thyroid, and behind the breastbone. The thymus has a thin outer covering called a capsule and consists of three types of cells. Thymic cell types include epithelial cells, lymphocytes, and Kulchitsky cells, or neuroendocrine cells.

  • Epithelial cells - tightly packed cells that give shape and structure to the thymus.
  • Lymphocytes - immune cells that protect against infection and stimulate an immune response.
  • Kulchitsky cells - hormone-releasing cells.

Each lobe of the thymus contains many smaller divisions called lobules. A lobule consists of an inner area called the medulla and an outer region called the cortex. The cortex region contains immature T lymphocytes. These cells have not yet developed the ability to distinguish cells of the body from foreign cells. The medulla region contains the larger, mature T-lymphocytes. These cells have the ability to identify self and have differentiated into specialized T lymphocytes. While T lymphocytes mature in the thymus, they originate from bone marrow stem cells. Immature T-cells migrate from the bone marrow to the thymus via the blood. The "T " in T lymphocyte stands for thymus-derived.

Thymus Function

The thymus functions chiefly to develop T lymphocytes. Once mature, these cells leave the thymus and are transported via blood vessels to the lymph nodes and spleen. T lymphocytes are responsible for cell-mediated immunity, which is an immune response that involves the activation of certain immune cells to fight infection. T-cells contain proteins called T-cell receptors that populate the T-cell membrane and are capable of recognizing various types of antigens (substances that provoke an immune response). T lymphocytes differentiate into three major classes in the thymus. These classes are:

  • Cytotoxic T cells - directly terminate antigens.
  • Helper T cells - precipitate the production of antibodies by B-cells and also produce substances that activate other T-cells.
  • Regulatory T cells - also called suppressor T cells, these cells suppress the response of B-cells and other T-cells to antigens.

The thymus produces hormone-like proteins that help T lymphocytes mature and differentiate. Some thymic hormones include thympoeitin, thymulin, thymosin, and thymic humoral factor (THF). Thympoeitin and thymulin induce differentiation in T-lymphocytes and enhance T-cell function. Thymosin increases immune responses. It also stimulates certain pituitary gland hormones (growth hormone, luteinizing hormone, prolactin, gonadotropin releasing hormone, and adrenocorticotropic hormone (ACTH)). Thymic humoral factor increases immune responses to viruses in particular.


The thymus gland acts to regulate the immune system through the development of immune cells responsible for cell-mediated immunity. In addition to immune function, the thymus also produces hormones that promote growth and maturation. Thymic hormones influence structures of the endocrine system, including the pituitary gland and adrenal glands, to assist in growth and sexual development. The thymus and its hormones also influence other organs and organ systems including the kidneys, spleen, reproductive system, and central nervous system.


  • SEER Training Modules, Thymus. U. S. National Institutes of Health, National Cancer Institute. Accessed 26 June 2013 (
  • Thymus Cancer. American Cancer Society. Updated 11/16/12 (