Why is the thymus important for T cells?

The thymus is a small specialized organ in the lymphatic system that regulates the development and selection of special white blood cells called T-cells, which play a key role in helping the immune system fight infection and disease. The thymus provides an optimum microenvironment that supports the development and maturation of highly diverse and functional T cells that are also tolerant to self. 

Precursor cells or thymocytes that originate in the bone marrow migrate to the thymus, where they undergo positive and negative selection successively. The positive selection process ensures that the T cells will be able to recognize foreign antigens while not attacking the body’s own tissues. The negative selection process eliminates thymocytes that strongly react with self-antigens in order to prevent the development of autoreactive T cells that could cause autoimmune diseases. 

On maturation, the T-cells enter the bloodstream and travel to the lymph nodes and other peripheral lymphoid organs where they initiate immune responses against pathogens. 

Without a properly functioning thymus, the immune system's ability to identify and combat infections and diseases would be severely compromised.