A condition that affects the immune system. The immune system is made up of cells, tissues, and organs that help the body fight infections and other diseases. There are many different types of immune system disorders, including immunodeficiency disease, autoimmune disorders, and allergic disorders. Immunodeficiency disease occurs when a part of the immune system is missing or not working properly. Autoimmune disorders occur when the immune system recognizes its own tissues as foreign and attacks them. Allergic disorders occur when the immune system overreacts to substances that are usually not harmful, such as pollen, molds, and certain foods. Cancers of the immune system (such as leukemia and lymphoma) are also immune system disorders.