Original Editor - User Name

Top Contributors - Lucinda hampton  

Introduction[edit | edit source]


The thymus gland, located behind your sternum and between your lungs, is only active until puberty. The thymus is the primary lymphoid organ for T lymphocyte development and maturation that mediates immune defense against foreign antigens, immune tolerance to self-antigens, and immune surveillance on tumor cells[1].

  • The thymus gland does not function throughout a full lifetime, but it has a big task when active of helping the body protect itself against autoimmunity, which occurs when the immune system turns against itself.
  • Before birth and continuing through childhood, the thymus is active in the production and maturation of T-lymphocytes (T cells), whose job it is to protects the body from certain threats, including viruses and infections.
  • The thymus produces and secretes thymosin, a hormone necessary for T cell development and production.

The thymus is unusual in that it is relatively large in infancy (weighing 25 g at birth) and reaches a maximal weight in adolescence between 12 and 19 years (35 g). It gradually involutes between 20 and 60 years of age , with progressive fatty replacement of the cellular components (15 g at 60 years of age). By age 75, the thymus is little more than fatty tissue. Fortunately, the thymus produces all of your T cells by the time you reach puberty.[2][3]

Immunity[edit | edit source]

T cell activation

The thymus is the organ primarily responsible for the production and maturation of T cells, functioning as the initial site of T cell immune maturation through positive and negative selection processes. T cells are so named as they mature in the thymus and B cells are named thus, as they mature in the bone marrow[4]. Both are a type of leukocyte.

T cells originate in the bone marrow and mature in the thymus. T cells, in the thymus, multiply and differentiate into helper, regulatory, or cytotoxic T cells or become memory T cells. They are next sent to peripheral tissues or circulate in the blood or lymphatic system. Upon stimulation by the appropriate antigen, helper T cells secrete cytokines, which stimulate the differentiation of B cells into plasma cells (antibody-producing cells). Regulatory T cells (as the name implies) act to control immune reactions. Cytotoxic T cells, which are activated by various cytokines, bind to and kill infected cells and cancer cells.[5]

Injury[edit | edit source]

The cells in the thymus are delicate and sensitive to changes in the external environment and subtle insults such as infections, chemo- and radiation-therapy, resulting in thymic injury and degeneration. The thymus has the capacity to self-regenerate, however it is often insufficient to restore intact thymic function. Thymic dysfunction leads to an increased risk of opportunistic infections, tumor relapse, autoimmunity, and adverse clinical outcome.[1]

  • An example of thymus injury is malnutrition. The thymus is indeed a barometer for nutrition and all anthropometric measurements and breastfeeding are associated with bigger thymus.[6]
  • After fibrofatty atrophy, the thymus can grow back at any time in life, particularly after periods of stress.[2] A Monash University research team has discovered how to stimulate the thymus gland so it grows back to full size and starts producing T-Cells again. They have proved it works in mice, and now trials are underway in cancer patients undergoing bone marrow patients[7].

Thymic Devolution[edit | edit source]


Thymic devolution is a hallmark of immunosenescence and inflammaging. With increasing age the thymus regresses, causing an increased susceptibility for disease. This decrease in thymus size and function leads to a decrease in circulating T cells and an alteration of their role. This modification in function can increase autoimmune diseases, bacterial and viral infections, and neoplasms. Re-establishing thymic function or intervening before involution could keep alive the immune system throughout adult life, accordingly the thymus is a current area of research with great promise.[4]

Exercise[edit | edit source]


A 2018 study (on older cyclist compared to a more sedentary group) concluded that many features of immunesenescence, which involves devolution of the thymus, may be driven by reduced physical activity with age. The impacts on riders’ immune system were marked.

  • In the older sedentary people, the output of new T cells from the thymus glands was low. The inactive older peoples’ thymus glands also were atrophied, compared to those of the younger group.
  • The aging cyclists, on the other hand, had almost as many new T cells in their blood as did the young people. Those who exercised also showed high levels of other immune cells that help to prevent autoimmune reactions and of a hormone that protects the thymus against shrinkage.

The researchers feel that the muscles are one of the sources of the hormone that protects the thymus.[8]

References[edit | edit source]

  1. 1.0 1.1 Duah M, Li L, Shen J, Lan Q, Pan B, Xu K. Thymus degeneration and regeneration. Frontiers in Immunology. 2021 Sep 1;12:706244. Available: (accessed 1.8.2022)
  2. 2.0 2.1 Radiopedia Thymus Available: (accessed 1.8.2022)
  3. Endocrine web Thymus Available: (accessed 1.8.2022)
  4. 4.0 4.1 Remien K, Jan A. Anatomy, Head and Neck, Thymus. InStatPearls [Internet] 2021 Jul 28. StatPearls Publishing. Available: (accessed 1.8.2022)
  5. Britannia T Cells Available: (accessed 1.8.2022)
  6. Prentice AM. The thymus: a barometer of malnutrition. British Journal of Nutrition. 1999 May;81(5):345-7. Available: (accessed 2.8.2022)
  7. Catalyst Regrowing the Thyroid gland Available: (accessed 2.8.2022)
  8. Duggal NA, Pollock RD, Lazarus NR, Harridge S, Lord JM. Major features of immunesenescence, including reduced thymic output, are ameliorated by high levels of physical activity in adulthood. Aging cell. 2018 Apr;17(2):e12750. Available: (accessed 2.8.2022)