The thymus is a specialized primary lymphoid organ that serves as a central training ground for the body’s defense system. Its function is to establish adaptive immunity by creating diverse and highly specific immune cells. These cells are prepared to fight foreign invaders without attacking the body’s own tissues. Understanding the structure and function of the thymus provides a clearer picture of how the body maintains health and defends itself.
Location and Appearance
The thymus is situated in the upper chest, known as the anterior superior mediastinum. It rests directly behind the sternum (breastbone) and is positioned between the two lungs, just above the heart. This central placement affords it a sheltered location within the torso.
Visually, the gland has a pinkish-gray coloration and a distinctly lobulated surface. It is composed of two symmetrical lobes joined in the midline. Each lobe is further divided into smaller sections called lobules, giving the organ its irregular shape.
A thin connective tissue capsule surrounds the structure, with extensions dividing the gland into lobules. Within each lobule, two regions are visible: a dense outer cortex and a lighter, central medulla. The thymus is notably large in infants and children, reaching its maximum size and activity around puberty, sometimes weighing up to 40 to 50 grams before beginning its decline.
Role in Immune System Function
The primary function of the thymus is the development and “education” of T-lymphocytes, or T-cells. These cells originate as immature precursors in the bone marrow and migrate to the thymus, where they are called thymocytes. This maturation process ensures T-cells are effective against foreign invaders while maintaining tolerance for the body’s own structures.
The education process begins in the cortex with positive selection. Thymocytes are tested to ensure their T-cell receptors can recognize self-molecules presented by the body’s cells, specifically the major histocompatibility complex (MHC). Only T-cells that successfully bind to these self-MHC molecules survive and proceed to the next stage.
Next, the cells move into the medulla for negative selection, a process designed to prevent autoimmune disease. During this phase, T-cells that react too strongly to the body’s own self-antigens are identified and eliminated, usually through programmed cell death. Specialized epithelial cells within the medulla present a wide range of proteins from other parts of the body to the developing T-cells.
This two-step filtering system ensures that T-cells exiting the thymus are both functional and tolerant. Functional T-cells are able to recognize an infected or abnormal body cell, while tolerant cells will not mistakenly attack healthy tissue. Once mature, these educated T-cells leave the thymus and enter the bloodstream to populate the lymph nodes and other lymphoid organs.
Changes Across the Lifespan
The thymus undergoes progressive shrinking over time, a process known as thymic involution. This regression starts surprisingly early, shortly after birth. The functional tissue, the thymic epithelial space, begins to decrease in size from the first year of life.
During childhood, functional tissue loss occurs at a rate of approximately three percent per year. After middle age (around 35 to 45 years old), the rate of decline slows to about one percent annually. As the functional tissue regresses, it is gradually replaced by non-functional fatty tissue.
This lifelong reduction in size and activity means the output of new, diverse T-cells steadily decreases with age. While the gland becomes largely non-functional in older adults, the immune system remains protected by the large reservoir of memory T-cells generated during youth. The decline in thymic activity contributes to the overall weakening of the immune system seen in advanced age.