The thymus gland is an endocrine organ of the immune system located anteriolateral to the trachea and in between the lungs. Its primary function is to build T lymphocytes for the body’s immune system; therefore, it is most important during childhood and puberty, when it reaches its maximum size. After puberty, it will begin to atrophy and shrink in size. Old age generally brings about hypotrophy of the thymus.

In children the thymus is grayish-pink in color and in adults it is yellow. On examination of a pediatric thymus it is very distinct; however, a geriatric thymus is difficult to locate and appears to be intertwined with surrounding fatty tissues. The thymus has two main components: lymphoid thymocytes and the thymic epithelial cells. Each lobe of the thymus is divided by trabeculae into lobules, which are further divided into nodules. Each lobule has an outer portion, called the cortex, and an inner portion, called the medulla. The cortex of the thymus is made up of lymphoid cells within a network of epithelial reticular cells. The medulla is made up of fewer lymphoid cells, and each are found in bundles called Hassall’s corpuscles. Both the cortex and the medulla are highly vascular, but the middle of each lobule has the least amount of blood supply.

The thymus has two distinct cell types: thymic stomal cells and hematopoietic cells. T-cells function by attacking specific antigens that correspond with their individual receptors. These unique receptors are genetically arranged during thymocyte maturation. The thymus should eliminate the T-cells that attack the body itself to prevent unnecessary disease and destruction.

There are three major categories of immune system diseases associated with the thymus gland: allergy, immunodeficiency, or auto-immune. Allergies result from hypersensitivity of the body’s defenses as an inappropriate or excessive immune response, where as immunodeficiency is classified as an abnormally low lymphocytes—in relation to the thymus, specifically T-cell underdevelopment and B-cell congenital defects. Severe Combined Immunodeficiency syndromes are group of rare congenital genetic diseases classified by combined T lymphocyte and B lymphocyte deficiencies; Human Immunodeficiency Virus, on the other hand, causes an Acquired Immunodeficiency syndrome by killing off T-cells. Autoimmune diseases are characterized by a hyperactive immune system that recognizes the body as a foreign pathogen and therefore attacks it. The thymus plays a role in preventing this type of response.

Image Caption: Thymus. Credit: Wikipedia