For most of history, the thymus has been a mysterious gland. It existed as early as the first century AD, but its role was not understood until much later. The ancient Greeks considered it the “Seat of Courage”. During the Renaissance, they thought it had no function. It wasn’t until the 1970s that its role in the immune system began to emerge.
Today we know that the thymus is an immune system organ. In the immune system, different white blood cells have different jobs. T lymphocytes or T cells are a type of white blood cell. In humans, the thymus is an organ that you can think of as a “boot camp” for infant T lymphocytes. It is where T lymphocytes (white blood cell immune soldiers) grow, develop, train and select, allowing them to mature and fight off infections and foreign invaders.
The “T” in T cells actually stands for the thymus, while the “B” in B cells refers to the bone marrow. All your white blood cells are produced in the bone marrow; only a specialized subset of these hematopoietic cells migrate from the bone marrow to the thymus, where they are “trained” to become T lymphocytes.
By the way, humans are not the only creatures with thymus glands – in fact, calf thymus and sometimes lamb thymus are prepared in a dish called sweetbread, which was once popular in the UK.
The location and size of the thymus
The thymus is a gland in the upper chest/lower neck area. The thymus is easily confused with the thyroid — a gland that is roughly the same but has a very different function. The thymus is a soft, pink-grey gland located behind the breastbone and between the lungs. Doctors call this area of the chest the mediastinum, and it’s packed with vital structures.
In humans, the thymus is not an organ that is usually visible or detectable from the outside. That said, a shadow of the thymus can sometimes be seen on an X-ray; however, lumps or bumps in the neck area are more likely to be caused by other causes, such as swollen lymph nodes or cysts. Rarely, a portion of the thymus is located further forward in the neck than it should be – called an ectopic cervical thymus.
Depending on your age, you may have at least remnants of the thymus gland, but in most cases, adults don’t have a truly active thymus gland. After puberty, the thymus begins to slowly shrink or shrink and is replaced by fat. Don’t worry though, as the thymus is widely believed to produce all the T cells you’ll ever need before that. Although the activity of the thymus gland appears to cease in adulthood, with rare exceptions, T lymphocytes continue to be produced in your body and replenished throughout your life.
The size and shape of the thymus can vary from person to person. Our thymus glands are relatively large as babies, weighing about 25 grams at birth. Maximum weight is reached between the ages of 12 and 19, averaging about 35 grams, and the thymus gland gradually shrinks from age 20 to age 60 as it is replaced by adipose tissue. By the age of 60, the average weight is about 15 grams.
The thymus is a ‘career counselor’ for baby’s white blood cells
Blood cells — red and white blood cells — come from stem cells in or originating from the bone marrow. During a child’s development, progenitor cells from the bone marrow migrate into the thymus, where thymocytes provide the appropriate environment, including cellular receptors and chemical signals, to nurture them properly.
When T cell progenitors migrate from the bone marrow into the thymus, they are called thymocytes, and signals and hormones from the thymus, including thymopoietin and thymosin, direct the development of thymocytes into adult T cells.
The thymus ensures that these thymocytes grow up with the correct “equipment” or markings on the outside of the cell. There is also a selection and elimination process. For example, at one of several checkpoints, about 95 percent of thymocytes were eliminated—only about 3 to 5 percent of thymocytes survived. Survivors differentiate into specialized (CD8+ or CD4+) lymphocytes and stay in a part of the thymus for about 10 days, where they learn to distinguish between “self” markers and foreign invader markers. After this complex process, T cells can leave the thymus and do various jobs in the immune system.
Enlargement can be a reaction to something or it can be the result of a disease process. Sometimes the thymus gland can enlarge during periods of stress or after treatment with certain drugs, such as chemotherapy and steroids. The thymus can also enlarge in a process called lymphoproliferative or autoimmune thymitis, which can be associated with conditions such as myasthenia gravis, systemic lupus erythematosus, rheumatoid arthritis, scleroderma, and Graves’ disease. Some of these diseases may also be associated with swollen lymph nodes.
When doctors evaluate the thymus on imaging, they try to distinguish whether the pattern is an overall thymic enlargement or one of malignancy, which tends to be more like a focal area of growth or an enlarged mass. Overall, thymic tumors are rare. It is estimated that there are only about 1.5 cases per million people per year in the United States, or about 400 cases per year.
Thymoma vs. Thymic Cancer: A thymoma is a tumor in which the tumor cells look similar to normal cells in the thymus. Thymoma grows slowly and rarely spreads beyond the thymus. In contrast, tumor cells in thymic cancer look very different from healthy thymocytes, grow rapidly, and often spread to other locations when the cancer is discovered. Thymic cancer is more difficult to treat than thymoma.
Myasthenia Gravis: Myasthenia gravis is an autoimmune disease associated with muscle weakness in the voluntary or skeletal muscles of the body. About 30% to 65% of patients with thymoma also have myasthenia gravis, arguably the most common autoimmune disease associated with thymoma. In myasthenia gravis, the body mistakenly produces antibodies to receptors on the surface of muscle cells that block the chemical signals that cause the muscles to move, resulting in severe muscle weakness.
People with myasthenia gravis feel tired easily and may have difficulty climbing stairs or walking long distances. Many people with thymoma have myasthenia gravis, but most people with myasthenia gravis do not have thymoma. Patients with myasthenia gravis and thymoma are usually treated with surgical excision. Resection may also be considered in patients with myasthenia gravis, even if they do not have a thymoma.
Hypopplasia or absence of the thymus: Conditions that prevent the normal development of the thymus can affect the immune system. DiGeorge syndrome is a disorder associated with genetic changes, usually the deletion of genetic information from a specific chromosome (chromosome 22). However, all different types of thymic abnormalities can occur in DiGeorge syndrome. However, most people with this syndrome have enough functional thymic tissue to develop healthy T cells. Complete absence of the thymus is possible, but appears to be relatively rare in patients with DiGeorge syndrome.
Thymus: Diagnosis and Surgical Management, edited by Kyriakos Anastasiadis, Chandi Ratnatunga. Springer Science & Business Media, June 7, 2007.