Immunity is, in part, a product of lymphoid tissue in the body, which includes the thymus, lymph nodes, tonsils, parts of the spleen and gastrointestinal tract, and bone marrow. Lymphocytes are divided into two groups. Immune system disorders occur when the immune response is inappropriate, excessive, or lacking. Immunodeficiency disorders occur when the immune system fails to fight tumors or invading substances. This causes persistent or recurrent infections, severe infections by organisms that are normally mild, incomplete recovery from illness or poor response to treatment, and increased incidence of cancer and other tumors.
Opportunistic infections are widespread infections by microorganisms that are usually controllable.
This deficiency may affect any part of the immune system. Inherited combined immunodeficiency affects both T lymphocytes and B lymphocytes. It is often fatal within the first year of life because there is no resistance to disease or infection. This is a desired part of treatment for disorders such as autoimmune disorders . It is used after organ transplantation to prevent transplant rejection .