The thymus is a small, bean-shaped organ located in the chest, behind the breastbone, and between the lungs. It is part of the body's lymphatic system, which helps to fight infection and disease.

The thymus produces two types of lymphocytes (white blood cells): T cells and B cells. These cells help to protect the body from bacteria, viruses, and other foreign substances.

The thymus is the largest at birth and continues to grow until puberty. After puberty, the thymus begins to shrink and is replaced by fat tissue. As people age, the thymus continues to shrink and may become barely detectable on an X-ray or CT scan.

The thymus is an important part of the immune system, but it is not well understood. Researchers are still studying how the thymus works and its role in health and disease.