Primary T-cell immunodeficiencies are rare inherited disorders that affect T-cell development and function. These disorders usually present in infancy or early childhood; however, the age of symptom onset may vary depending on the underlying gene defect. Although T-cell immune responses may be selectively affected, abnormal B-cell function often is associated, in part because of concomitant intrinsic B-cell defects but also because production of most antibodies is dependent on T-cell help.