An effective immune system requires that any antigens that are not native to the body be quickly recognized and destroyed, and that none of the antigens native to the body be identified as foreign. Excesses in the latter constitute the autoimmune diseases. Deficiencies in the body's ability to recognize antigens as foreign or a diminished capacity to respond to recognized antigens constitute the immunodeficiency syndromes.

There are many causes associated with immunodeficiencies. Primary immunodeficiencies are inherited conditions in which specific genes or gene families are corrupted by mutations or chromosome deletions. These syndromes are discussed elsewhere in this volume. Secondary immunodeficiencies are acquired conditions that may result from infections, cancers, aging, exposure to drugs, chemicals or radiation, or a variety of other disease processes.

Bacteria, viral, fungi, protozoa, and even parasitic infections can result in specific deficiencies of B cells, T cells, macrophages, and...