The thymus

The thymus is a small, irregular-shaped gland in the top part of the chest, just under the breastbone and between the lungs. It is located in an area of the body 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 lymphocytes and fat tissue.

The thymus changes in size as you get older. It is large in newborns and toddlers. It is biggest during puberty then slowly begins to shrink as adulthood approaches.

The thymus is most active during childhood and youth. By late adulthood, most of the thymus is made up of fat tissue.

What the thymus does

The thymus makes T cells (T lymphocytes) that travel throughout the body to help fight infection, disease and foreign substances. The thymus also makes hormones to help T cells develop and keep the immune system working properly.

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 help the immune system. T cells also travel to lymph nodes and the spleen where they continue to mature.

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