Congenital deficiencies of the immune system occur in children or adults and can cause severe or recurrent infections. The overall incidence of these immunodeficiency diseases is estimated at approximately 1 in 10,000, excluding selective immunoglobulin A deficiency, but this estimation is based on population studies, not hospital or clinic populations. The majority of immune defects involve antibody production; these immune deficiencies are found more often in adults than infants and children. In an allergy practice, recurrent infections are common, and determining if an immune defect is likely to be present can be problematic. Some guidelines concerning the clinical presentation and laboratory evaluation and treatment options can aid the practicing clinician.