When a system errs by failing to protect the host from disease-causing agents or from malignant cells, the result is immu-nodeficiency. Immunodeficiency diseases and syndromes are the causes of significant mortality and morbidity, as well as a source of extremely valuable information about the physiology of the human immune system.
Immunodeficiency can occur in T cells, B cells, comple-ment, and phagocytes—the major components of the immune system. A functional defect of the immune system is suspected when a patient:
· Has unusual frequency of infections with common or opportunistic microorganisms;
· Has unusually severe infections; and
· Is unable to eradicate infections with antibiotics to which the microorganisms are sensitive. Recurrent infections with certain viruses, protozoa, and fungi indicate a T-cell deficiency, whereas recurrent infections with pyogenic bacteria (such as staphylococci) indicate a B-cell deficiency.
Immunodeficiency disorders can be classified as (a) primary immunodeficiencies or (b) secondary immunodeficiencies.