Disorders in which the defences of the immune system are unable to fight infection and tumours. Immunodeficiency disorders can be because of a congenital or inherited defect or can be the result of an accumulated disease. The result is constant or recurrent infection, including infections with organisms that would not generally cause disease, and undue susceptibility to some forms of cancer. Opportunistic infections include fungal infections, pneumonia, and frequent herpes simplex infections.
Inherited or congenital deficiencies can arise in either of the two prongs of the adaptive immune system – humoral or cellular immunity or in both.
Deficiencies of the humoral system include agammaglobulinaemia and hypogammaglobulinaemia. The former can cause few or no symptoms, depending on the extremity of the deficiency, but agammaglobulinaemia may be fatal if it is not treated with immunoglobulin.
Congenital deficiencies of T-lymphocytes can lead to problems such as widespread and constant candidiasis (thrush). A combined deficiency of both cellular and humoral components of the immune system, called severe combined immunodeficiency (SCID), is often fatal in the first year of life unless treatment can be administered by bone marrow transplant or stem cell.
Acquired immunodeficiency can be caused by disease processes (like infection with HIV, which leads to AIDS). It can also be brought on by damage to the immune system as a result of its suppression by drugs, either intentionally as in treatment for autoimmune diseases or after organ transplantation, or as a side effect of treatment for another condition. Many cancers and extreme malnutrition can also lead to immunodeficiency. Mild immunodeficiency occurs through a natural decline in immune defences with age.