Table 1.

Examples of severe immunodeficiency disorders^*

System involved	Typical clinical syndrome	Example of genetic defect
B lymphocyte	Recurrent bacterial infection due to defective antibody production	B cell cytoplasmic tyrosine kinase
		CD40 ligand
T lymphocyte	Severe bacterial, viral, and fungal infection due to defective humoral and cellular immunity	Interleukin 2 receptor γ chain
		Adenosine deaminase
Neutrophil	Severe bacterial infection due to defective phagocytosis	Cytochrome b
		β2 integrin
Macrophage	Extreme susceptibility to infection with environmental mycobacteria	Interferon γ receptor
Complement	Recurrent Neisseria infection	Terminal complement components

^*Almost 100 severe deficiency disorders have been identified. Each is caused by a rare mutation of a single gene. Different mutations in the same gene may cause subtle variations in clinical phenotype. Mutations of different genes may lead to similar clinical syndromes if they disrupt a common immune pathway.