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
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.