Immunodeficiency diseases result from a failure of one or more components of the immune system and may involve the absence or malfunction of organs, cells or proteins of the immune system. Where the immunodeficiency results from a direct defect within the immune system, the disease is classified as a primary immunodeficiency (PID). All PIDs are inherited or have a geneticcomponent. Most, but not all, primary deficiencies are congenital, that is are present from birth, although some do not manifest themselves until later in life. Secondary immunodeficiencies (SIDs) arise as a consequence of other conditions. For example, some viruses are associated with an immunosuppression, which may be transient, as for example in measles, or permanent, as in HIV infection. Conditions that can give rise to secondary immunodeficiency are listed in Table 5.1.
Patients with immunodeficiency diseases invariably suffer infections more frequently and with increased severity although the type of infection depends to a large extent on the nature of the immune deficit. Some general rules to guide diagnoses are given in Table 5.2. Other types of disorder may also arise as a consequence of a PID. So, for example, deficiencies of some complement proteins may lead to systemic lupus erythematosus (SLE), an autoimmune disorder. Certain types of cancer, such as non-Hodgkins lymphoma, a tumor of the lymph glands, and Kaposi’s sarcoma, a tumor derived from blood vessels and which frequently shows in the skin, are also more frequent in the immunodeficient patient, because these tumors are linked to infections with certain viruses.