The thymus is a small organ in the front part of the chest under the breastbone. It runs from the lower neck to above the level of the heart. During development in the womb and in childhood, the thymus makes a type of white blood cell called a T-lymphocyte. T-lymphocytes are important to the immune system. They grow in the thymus and then travel to the lymph nodes where they help protect the body against infections and cancer.
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