There are 264 primary immunodeficiencies (PIDs), most of which are rare. They are caused by complement deficiencies, defects in phagocyte function, impaired T-cell function, and/or impaired B-cell function with antibody deficiencies. Most patients with PIDs will present, at varying ages, with frequent infections. These infections can be common respiratory tract infections such as otitis media or pneumonia, or they can be unusual bacterial, fungal, or parasitic infections. Neonatal screening for severe combined immunodeficiency syndrome, one of the most common and serious PIDs, is now performed in most US states, but many PIDs manifest and are detected after birth. Clinicians should be alert for PIDs when patients have unusual or frequent infection and perform a diagnostic evaluation. After ruling out HIV and hepatitis C infection, the next step is to obtain a complete blood count, immunodeficiency panel, and immunoglobulin and complement levels. If results are abnormal, or if a PID is suspected clinically but the diagnosis is not clear, prompt referral to an appropriate subspecialist is indicated. Some PIDs can be managed with stem cell transplantation, and transplantation before the first serious infection is associated with better outcomes. In addition, antimicrobial prophylaxis is indicated for many PIDs patients to prevent opportunistic infections.
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