The thymus is a specialized immune system organ located in the upper chest, behind the breastbone and between the lungs. It is a primary lymphoid organ central to the development and maturation of T-lymphocytes, or T-cells, which are essential for the body’s defense.
Key Hormones of the Thymus
The thymus produces several hormones influencing immune function. Thymosin is a prominent hormone that stimulates T-cell production. Thymosin alpha-1, a member of this family, also stimulates T-cell production. Another, thymosin beta-4, is a peptide found in many tissues involved in repair and regeneration.
Thymopoietin also contributes to T-cell generation. Thymulin, a nonapeptide hormone, is produced by the thymus and requires zinc for its activity. This hormone participates in T-cell differentiation and enhances the actions of T and natural killer (NK) cells.
Their Role in Immune System Development
T-cells originate as precursor cells in the bone marrow and travel to the thymus for specialized training. Within the thymus, these cells undergo T-cell education, ensuring they recognize foreign invaders while tolerating the body’s own tissues.
This education involves positive and negative selection. Positive selection occurs as immature T-cells interact with self-molecules called Major Histocompatibility Complex (MHC) on thymic cells. Only T-cells that bind to these MHC molecules with moderate affinity receive survival signals and continue development. T-cells failing this step are eliminated.
Following positive selection, T-cells proceed to negative selection. Here, T-cells strongly reacting to the body’s own self-antigens presented by MHC molecules are removed. This prevents T-cells from mistakenly attacking healthy body tissues, averting autoimmune conditions. Thymic hormones, like thymosin, guide this maturation, ensuring T-cells can perform their immune duties. Healthy T-cell development supports adaptive immunity, allowing the body to fight infections and abnormal cells.
Thymus and Immune Health Over Time
The thymus undergoes significant changes throughout life, a process known as thymic involution. While large in infants and children, it decreases in size after puberty. This shrinking involves the gradual replacement of active thymic tissue with fatty tissue. While often associated with puberty, thymic involution can begin in the first year of life, with a more pronounced decline in adulthood.
Despite this age-related reduction, the thymus continues to produce some hormones. Its early activity has lasting effects on the immune system’s T-cell diversity. However, the progressive shrinking and reduced output of new T-cells can decrease the diversity of the T-cell repertoire in later life. This reduction is associated with increased susceptibility to infections in older adults.