Hyper-IgM Syndrome By James Fernandez, MD, PhD, Cleveland Clinic Lerner College of Medicine at Case Western Reserve University Reviewed / Revised Jan 2023 View Patient Education Diagnosis Treatment Hyper-IgM syndrome is an immunoglobulin (Ig) deficiency characterized by normal or elevated serum IgM levels and decreased levels or absence of other serum immunoglobulins, resulting in susceptibility to bacterial infections.