The thymus is a primary lymphoid organ that plays a unique role in the body’s defense system. It is where immune cells develop and mature, helping the body protect itself from foreign invaders.
Where is the Thymus Located and What Does It Look Like?
The thymus is located in the upper chest, specifically behind the sternum (breastbone) and between the lungs. It rests in front of the heart and great vessels. The gland has an asymmetrical, flat shape and is composed of two distinct lobes that typically meet in the upper midline.
The thymus generally appears pinkish-grey. Its surface has a soft, lobulated texture, with many small bumps called lobules. Each lobe is encased by a capsule and divided into smaller lobules, which are comprised of an outer cortex and a central medulla.
The Thymus’s Role in Immunity
The thymus is primarily responsible for the development and maturation of T-lymphocytes, commonly known as T cells. These T cells originate as immature white blood cells in the bone marrow and then travel to the thymus for their specialized training. Within the thymus, T cells undergo a maturation process, which involves “positive selection” to ensure they can recognize the body’s own cells and “negative selection” to eliminate T cells that would mistakenly attack the body’s healthy tissues.
This process, often referred to as immune education, enables the body to distinguish between its own components and potentially harmful foreign invaders like bacteria and viruses. Once T cells are fully mature, they leave the thymus and circulate throughout the bloodstream and lymphatic system. They then take on various roles, differentiating into specialized types such as helper, regulatory, or cytotoxic T cells, or becoming memory T cells, all working to identify and neutralize pathogens and abnormal cells. The thymus also produces hormones like thymosin and thymulin, which support T cell production and specialization.
How the Thymus Changes Over Time
The thymus undergoes changes throughout a person’s life, beginning to develop before birth. It is largest and most active during infancy and childhood, reaching its maximum weight, around 35 grams, during adolescence. After puberty, the thymus gradually begins to shrink in a process called involution, where its cellular components are replaced by fatty tissue.
This shrinking process starts as early as the first year of life, with a tissue loss rate that slows after middle age. By age 75, the thymus may be largely composed of fatty tissue. Despite this age-related reduction in size, the thymus continues to contribute to immune function throughout adult life, though its output of new T cells decreases.