The Anatomy of the Thymus

Organ located in the chest produces immune system T-cells

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The thymus is a lymphatic system organ located in the chest, behind the sternum (breastbone). The thymus plays an important role in the immune system by creating lymphocytes called T cells. These cells find and destroy pathogens like bacteria circulating in the bloodstream.

Although the thymus is sometimes called a gland, it is not structured like a gland. Also, it should not be confused with the thyroid gland, which is located in the neck.

Human Thymus Anatomy
Nerthuz / Getty Images


The thymus is located at the top of the sternum (breastbone), near the collarbone. It resides between the sternum and the aortic arch. At birth, the thymus measures approximately 1 to 2 inches wide by one-half inch thick. The organ grows in size throughout childhood, until it begins to shrink in adolescence.

The thymus is encapsulated by a wall made of collagen-type tissue. Inside, the organ is divided into two main lobes with irregular lobules (sub-lobes), each of which contains several structures and cell types:

  • Cortex: Nearest the organ’s wall, the cortex region contains developing T cell lymphocytes
  • Medulla: A region near the center of each lobule, the medulla holds fully developed T cells
  • Epithelioreticular cells: These cells create walls that divide the organ into a latticework of sections that hold developing and mature T cells
  • Blood vessels: The capsule and lobular walls contain blood vessels to supply oxygen to the organ’s tissues
  • Lymphatic vessels: Similar to blood vessels, lymphatic vessels carry lymphatic fluid through the body’s lymph system, including the thymus
  • Macrophages: These immune system cells destroy T cells that have not developed properly

Anatomical Variations

The shape of the thymus can vary widely in infants, sometimes stretching above the clavicle. Infants can be born with an enlarged thymus that puts pressure on the trachea (windpipe), heart, or other structures. It is not always recommended that the thymus be removed in these cases, as it can have a negative effect on immune system development.


The only purpose of the thymus is to produce white blood cells called T lymphocytes (T cells). They are called T cells because they are primarily produced in the thymus. The thymus produces some T cells before birth and continues the process from birth through adolescence.

T cells come in several varieties that perform various roles in the immune response. The most common types of T cells and their roles are:

  • T4 or CD4 cells: Alert other white blood cells to pathogens, so they can be destroyed
  • T8 or CD8 cells: Control the overall immune system response by suppressing the activities of other white blood cells
  • Killer T cells: This specific type of CD8 cell recognizes and destroys foreign cells, cancer cells, and those infected with a virus.

Associated Conditions

Although the thymus stops producing T cells in adolescence and gradually shrinks away, it can be affected by cancer. The two main types of cancer that can arise in the thymus are:

  • Thymoma: A tumor of the thymus
  • Thymic cancer: A type of thymoma that often spreads (metastasizes)

Thymoma and thymic cancer are rare. The risk of developing cancer of the thymus increases if a person has one of these other medical conditions:

  • Myasthenia gravis: A chronic autoimmune and neuromuscular disease
  • Lupus: An autoimmune disease that causes chronic, systemic (body-wide) inflammation
  • Rheumatoid arthritis: An autoimmune disease that causes chronic inflammation of the joint tissues


Doctors may use imaging tests, including ultrasound, computerized tomography (CT), or magnetic resonance imaging (MRI) to evaluate the size of an infant’s thymus gland. The same tests may be used in conjunction with lab (blood) testing in suspected thymoma or thymic cancer.

When they suspect an autoimmune disorder, doctors may order sequences of tests to chart CD4 levels in the bloodstream. Higher or lower than normal CD4 counts can indicate acute infection, cancer, immune system diseases like HIV/AIDS, and other conditions.

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By Elizabeth Hanes, BSN, RN
Elizabeth Hanes, BSN, RN, is a nurse who has been writing health and wellness information for the public for nearly a decade.