Anatomy Organs The Anatomy of the Thymus Organ located in the chest produces immune system T-cells By Elizabeth Hanes, BSN, RN Elizabeth Hanes, BSN, RN Facebook LinkedIn Twitter Elizabeth Hanes, BSN, RN, is a nurse who has been writing health and wellness information for the public for nearly a decade. Learn about our editorial process Updated on April 09, 2022 Medically reviewed by Brian H. Wetchler, DO Medically reviewed by Brian H. Wetchler, DO Brain H. Wetchler, DO, is board-certified in internal medicine and works in private practice at Woodmere Medical Associates. Learn about our Medical Expert Board Print Table of Contents View All Table of Contents Anatomy Function Associated Conditions Tests 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. Nerthuz / Getty Images Anatomy 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 lymphocytesMedulla: A region near the center of each lobule, the medulla holds fully developed T cellsEpithelioreticular cells: These cells create walls that divide the organ into a latticework of sections that hold developing and mature T cellsBlood vessels: The capsule and lobular walls contain blood vessels to supply oxygen to the organ’s tissuesLymphatic vessels: Similar to blood vessels, lymphatic vessels carry lymphatic fluid through the body’s lymph system, including the thymusMacrophages: 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. Function 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. Understanding the Function of the Thymus 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 thymusThymic 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 Tests 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. Sources Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy. Encyclopaedia Britannica. Thymus gland. Remien K, Jan A. Anatomy, head and neck, thymus. StatPearls. Updated March 20, 2019. U.S. National Cancer Institute. Thymus. U.S. National Cancer Institute. Thymoma and thymic carcinoma treatment (adult) (PDQ®)–patient version. Updated March 5, 2020. U.S. National Library of Medicine, MedlinePlus. T-cell count. Updated March 4, 2020. U.S. National Library of Medicine, MedlinePlus. Thymus cancer. Updated November 19, 2019. Yale Medical School. Thymus. Zdrojewicz Z, Pachura E, Pachura P. The thymus: A forgotten, but very important organ. Adv Clin Exp Med. 2016;25(2):369–375. doi:10.17219/acem/58802 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. See Our Editorial Process Meet Our Medical Expert Board Share Feedback Was this page helpful? Thanks for your feedback! What is your feedback? Other Helpful Report an Error Submit