The thymus is a small organ that plays an important role in the body’s defense system. It is responsible for the development and maturation of specific immune cells.
Where the Thymus Sits
The thymus is located in the upper chest, behind the breastbone and in front of the heart. It sits in the superior mediastinum. The organ has two lobes and is often described as resembling a small, butterfly-shaped structure. It measures approximately 30 to 40 millimeters in length and 25 to 35 millimeters in width.
The Thymus and Immune System Training
The primary function of the thymus involves the maturation and “training” of T-lymphocytes, commonly known as T-cells. Immature T-cells, called thymocytes, migrate from the bone marrow to the thymus. Within the thymus, these cells undergo a selection process in two main areas: the cortex and the medulla.
This training ensures that T-cells can effectively recognize foreign invaders like bacteria and viruses, while also learning to tolerate the body’s own healthy cells. Only T-cells that pass both positive and negative selection processes exit the thymus. These mature T-cells then circulate throughout the body, ready to respond to specific pathogens, forming a central part of the adaptive immune system.
How the Thymus Changes Over Time
The thymus changes over time, reaching its largest size and highest activity during childhood and puberty. After puberty, the thymus begins a process called involution, where it gradually shrinks and is replaced by fatty tissue. By age 65, the ability to produce new T-cells is significantly diminished.
This age-related decline in thymic function has implications for the immune system in later life. An involuted thymus can lead to a reduced capacity to generate new T-cell populations, potentially increasing susceptibility to infections and impacting the effectiveness of vaccines in older adults.
When the Thymus Doesn’t Work Right
Dysfunction of the thymus can impact the immune system. In some congenital conditions, such as DiGeorge syndrome, children may be born with a partially developed or absent thymus, leading to severe immunodeficiency due to a lack of mature T-cells. This makes them highly vulnerable to infections.
The thymus can also be involved in autoimmune diseases like myasthenia gravis, where it may remain enlarged in adults and potentially contribute to the disease’s progression. Other conditions, such as thymic tumors or certain lymphatic cancers, can also affect the thymus, causing symptoms from its growth and pressure on surrounding structures.