The immune system consists of a network of cells and chemical mediators. The immune system consists of a network of cells and chemical mediators. The main function of this system is to protect the host against infections, tumors, foreign body, etc. The major components of the network are the leukocytes, the immunoglobulins and complement. They all interact with each other to keep the delicate balance of immunity. The functions of the immune system that can be altered as a result of exposures to toxins are phagocytosis, humoral immunity and cellular immunity. The adverse responses of the immune system to environmental toxins can be divided into four different groups. A large number of drugs have been associated with a variety of autoimmune phenomena. The chapter divides into three main sections: the normal immune system, lab assessment of the immune system, types of hypersensitivity and effects of toxins on the immune system.