The Thymus

The Thymus

The thymus is a specialized primary lymphoid organ of the immune system. Within the thymus, T cells or T lymphocytes mature. T cells are critical to the adaptive immune system, where the body adapts specifically to foreign invaders. The thymus is located in the upper anterior part of the chest cavity, just behind the sternum.

Anatomy

The thymus is composed of two identical lobes and is located anatomically in the anterior superior mediastinum, in front of the heart and behind the sternum. Each lobe of the thymus is divided into a central medulla and a peripheral cortex, which is surrounded by an outer capsule.

Cortex

The cortex is the outer region of the thymus and is densely packed with immature T cells, known as thymocytes. The cortex is the site of positive selection, where thymocytes that can bind to self-major histocompatibility complex (MHC) molecules are selected for survival.

Medulla

The medulla is the inner region of the thymus and contains fewer thymocytes. It is the site of negative selection, where thymocytes that bind too strongly to self-antigens are eliminated. This process is crucial for preventing autoimmunity.

Function

The primary function of the thymus is the production and maturation of T lymphocytes (T cells). T cells are essential for the adaptive immune response, which is the body's way of fighting off specific pathogens.

T Cell Maturation

T cell maturation in the thymus involves several stages:

• Positive Selection: Occurs in the cortex, where thymocytes that can recognize self-MHC molecules are selected to survive.
• Negative Selection: Occurs in the medulla, where thymocytes that react strongly to self-antigens are eliminated.
• Differentiation: Surviving thymocytes differentiate into various types of T cells, including helper T cells, cytotoxic T cells, and regulatory T cells.

Development

The thymus is most active during the neonatal and pre-adolescent periods. After puberty, the thymus begins to atrophy and is replaced by adipose tissue, although it continues to produce T cells throughout adult life.

Clinical Significance

Thymic Disorders

• Thymoma: A tumor originating from the epithelial cells of the thymus. It can be associated with myasthenia gravis and other autoimmune diseases.
• Thymic Hyperplasia: An enlargement of the thymus that can occur in response to stress or infection.
• DiGeorge Syndrome: A genetic disorder resulting in thymic hypoplasia, leading to immunodeficiency.

Role in Autoimmunity

The thymus plays a crucial role in preventing autoimmunity by eliminating self-reactive T cells. Failure in this process can lead to autoimmune diseases.

See Also

• Immune System
• T Cell
• Autoimmunity
• Lymphoid Organs

External Links

• [Thymus on Wikipedia](https://en.wikipedia.org/wiki/Thymus)