Our immune system fights infections and cancer cells with a type of white blood cell, the T cell. T cells are made in an organ called the thymus. Immature blood cells made from blood stem cells in the bone marrow travel through the blood stream to the thymus. Inside the thymus, over the course of approximately two weeks, a network of different cells in part chemical signals that affect the fate of the immature T cells. They are guided through a process of development, selection, and also elimination to become mature T cells. Firstly, the immature T cells are instructed to develop receptors. At the next stage, those cells with receptors that can respond to the type of proteins that want to useful to our immune system are signal to survive. The rest are eliminated. After this stage, some of the developing T cells may recognize proteins from the healthy cells of our own body, which can be dangerous and cause auto-immunity, where they may target those cells and tissues. In the final stage, these cells are eliminated. The remaining now matured T cells, which have passed through the selection process, enter the blood stream and become part of our immune system. In fact, as little as 1% reach this stage, and are now ready to protect against harmful infection and disease, but not the healthy cells of our own body. From a young age, our thymus degenerates, getting smaller. Throughout our lives, fewer and fewer T cells are made, reducing our protection against new infections and cancer, especially in old age. Finding ways to regenerate the thymus could help to boost the immune system when this happens, and could also lead to treatments for a number of conditions affecting both the thymus and the immune system. www.thymus.org