The functions of the immune system are mainly of the following three categories: immune defense, immune surveillance and immune self-stabilization. Immune defense is mainly the intrinsic and adaptive immunity of human skin, mucous membranes, and various immune cells, effector cells, killer cells, etc. It can effectively prevent the invasion of external pathogens, and also remove pathogens and other harmful substances that have already invaded the body. Immune surveillance such as myocytes can readily detect, and remove pathogenic components such as tumors, senescent cells and apoptotic cells that appear in the body. Immune self-stabilization is the achievement of homeostasis of the immune system’s internal environment through autoimmune tolerance and immune regulation, so that its function is not altered. Once immune homeostasis is altered and immune function is disrupted, a number of autoimmune and allergic diseases can occur. The immune system is too strong, too low or deficient also causes corresponding diseases, such as hypersensitivity reactions or immunodeficiency diseases, etc.