The immune system is a network of biological systems that protects an organism from diseases. Immunotherapy, also called biological therapy, treats diseases by activating or suppressing the immune system. Immunotherapy uses substances to stimulate or suppress the immune system to help the body fight cancer, autoimmune disorder and other diseases. Immunotherapeutics have transformed the treatment of malignant diseases and also afford major opportunities for treating diseases beyond cancer. Apart from cancer treatment, immunotherapy reduces an abnormal immune response in autoimmune diseases; in organ transplants, a normal immune response is reduced to prevent rejection of transplanted organs or cells; In diseases caused by complement activation, the complement system is inhibited; and in allergic diseases, tolerance to the allergen develops. Immunotherapy in benign hematological diseases is reviewed here.