Definition
The thymus is a specialized primary lymphoid organ of the immune system. It is essential for the development and maturation of T lymphocytes (T cells), which are crucial for adaptive immunity. The thymus is most active during childhood and gradually shrinks and becomes less active with age.
Structure of Thymus
The thymus is a bilobed organ located in the upper anterior part of the chest, just behind the sternum and in front of the heart. It can be divided into several components:
1. Lobes: The thymus consists of two lobes connected by an isthmus. Each lobe is further divided into lobules by connective tissue septa.
2. Cortex: The outer region of each lobule, densely packed with immature T cells (thymocytes) and epithelial cells.
3. Medulla: The inner region of each lobule, containing fewer thymocytes and more mature T cells, as well as a higher number of epithelial cells and Hassall’s corpuscles.
4. Capsule: A thin layer of connective tissue that surrounds the thymus and extends into the organ to form the septa.
The thymus is richly supplied with blood vessels, lymphatic vessels, and nerves, allowing it to interact with other parts of the immune system.
 Functions of Thymus
The primary functions of the thymus are related to the development and education of T cells, which are vital for adaptive immunity. The key functions include:
 T Cell Maturation
T cells originate from hematopoietic stem cells in the bone marrow and migrate to the thymus to mature. The maturation process involves several stages:
1. Proliferation: Immature T cells, or thymocytes, proliferate in the thymic cortex.
2. Positive Selection: Thymocytes that can recognize self-major histocompatibility complex (MHC) molecules with moderate affinity are selected for further maturation.
3. Negative Selection: Thymocytes that bind too strongly to self-antigens are eliminated to prevent autoimmune reactions.
4. Differentiation: Surviving thymocytes differentiate into various types of mature T cells, including helper T cells (CD4+), cytotoxic T cells (CD8+), and regulatory T cells.
 Central Tolerance
The thymus plays a crucial role in establishing central tolerance, a process that ensures the immune system does not attack the body’s own tissues. By eliminating self-reactive T cells during negative selection, the thymus helps prevent autoimmune diseases.
 Hormone Production
The thymus produces several hormones, such as thymosin, thymopoietin, and thymulin, which promote T cell development and regulate immune functions.
 Disorders of Thymus
Disorders of the thymus can disrupt immune function and lead to various health problems. Major thymic disorders include:
 Thymic Hypoplasia/Aplasia
Thymic hypoplasia or aplasia is a congenital condition where the thymus is underdeveloped or absent. This can lead to severe immunodeficiency as the body cannot produce sufficient T cells. Examples include:
– DiGeorge Syndrome: A genetic disorder caused by a deletion on chromosome 22, resulting in thymic hypoplasia and a range of other abnormalities such as heart defects, facial dysmorphisms, and hypocalcemia.
 Thymoma
Thymomas are tumors originating from the epithelial cells of the thymus. They are usually benign but can be malignant. Thymomas are often associated with autoimmune diseases, most notably myasthenia gravis. Symptoms may include chest pain, cough, and difficulty swallowing.
 Thymic Carcinoma
Thymic carcinoma is a rare, aggressive cancer of the thymus. Unlike thymomas, thymic carcinomas are more likely to spread to other parts of the body. Symptoms are similar to thymomas but may be more severe and include weight loss and fatigue.
 Myasthenia Gravis
Myasthenia gravis is an autoimmune disease characterized by weakness and fatigue of voluntary muscles. It is often associated with thymic abnormalities, including thymoma and thymic hyperplasia. The thymus is believed to play a role in the production of autoantibodies that target acetylcholine receptors at the neuromuscular junction.
 Thymic Hyperplasia
Thymic hyperplasia refers to an enlargement of the thymus, often associated with autoimmune diseases like myasthenia gravis and systemic lupus erythematosus. It can be asymptomatic or present with symptoms related to the underlying autoimmune condition.
Conclusion
The thymus is a vital organ for the development and maturation of T cells, which are essential for the adaptive immune system. It plays a critical role in establishing central tolerance to prevent autoimmunity. Disorders of the thymus, such as thymic hypoplasia, thymoma, thymic carcinoma, and thymic hyperplasia, can significantly impact immune function and lead to a range of health issues. Understanding the structure, functions, and potential disorders of the thymus is crucial for diagnosing and managing conditions related to immune system dysfunction.
