Abstract
In this chapter, we will first discuss about the impact and mechanisms of immunological tolerance, which is differentiated into central and peripheral tolerance. Failure of tolerance is a main contributor to autoimmune and allergic disease. One major mechanism of peripheral tolerance is the action of T REG cells that suppress other self-reactive lymphocytes. Then we will discuss the molecular and cellular basis of transplantation immunology and distinguish between hyperacute, acute and chronic reactions of the host’s immune system against a graft obtained from a different person. Finally, we will explain the molecular mechanisms of immunosuppressive drugs, which are used for the prevention of graft rejections and will discuss their side effects.