The thymus
The thymus is a small, irregular-shaped gland in the top part of the chest, just under the breastbone. It is in the area between your lungs (called the mediastinum). The thymus is part of both the lymphatic system and the endocrine system.
Structure of the thymus
The thymus is divided into 2 main parts – a right lobe and a left lobe. Each lobe is divided into smaller sections called lobules that give the thymus its bumpy appearance. Each lobule is made up of a centre part (called the medulla) and an outer layer (called the cortex). A thin covering (capsule) surrounds and protects the thymus.
The thymus is mainly made up of
epithelial cells,
immature and mature
The thymus is large in newborns and toddlers, and is most active during childhood and adolescence. It grows to its largest size during puberty. After puberty, the thymus slowly shrinks as the epithelial cells begin to be replaced by fat tissue.
By late adulthood, most of the thymus is made up of fat tissue.
What the thymus does
The thymus makes T cells (called T lymphocytes) that travel throughout the body to help fight infection, disease and foreign substances.
Lymphocytes travel from the bone marrow to the thymus, where they mature into T cells. Once T cells mature, they are able to leave the thymus and enter the blood so they can work with the immune system. T cells also travel to lymph nodes and the spleen where they continue to mature.
The thymus also makes
hormones
and other substances that help T cells develop and keep the
- thymopoietin
- thymulin
- thymosin
- thymic humoral factor
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