Summary

• Can be primary (congenital) or secondary. It presents in childhood and adulthood, and can affect both sexes.
• Primary disease (most common cause being common variable immunodeficiency [CVID]) may have a delay of several years between clinical presentation and reaching a diagnosis.
• Secondary disease causes include malabsorption, nephrotic syndrome, haematological malignancy, and chemotherapy.
• Patients are predisposed to recurrent upper and lower respiratory tract infections.
• Treatment is tailored to specific causes and individual patient needs, and includes intravenous or subcutaneous immunoglobulin replacement.
• Severe combined immunodeficiency (SCID) is a medical emergency and suspected cases require immediate specialist centre referral for diagnosis and treatment.