**4. Physiology of the thymus**

The thymus plays the main role in the development of the immune system during infancy and childhood [15]. The thymus gland has the responsibility for implying differentiation of CD4 and CD8 T cells. It also proliferates clones of mature T cells for entering the lymph flow and developing immune tolerance resulting in prevention of autoimmune diseases. On the other hand, the thymus is involved in secretion of hormones such as thymulin, thymosin alpha 1, and thymopentin [2–4].
