Primary immunodeficiency diseases (PID) comprise a heterogeneous group of disorders characterized by impaired host defense mechanisms. Increased susceptibility to infection is central clinical feature, and immunoglobulin replacement therapy is widely used for infection prevention. Although meta-analyses have suggested that maintaining serum IgG trough levels above 1,000 mg/dL reduces the risk of pneumonia, the optimal IgG target is thought to vary among individuals. Furthermore, the relationship between IgG levels and other immune dysregulation remains unclear. As therapeutic options have expanded including subcutaneous formulations, there is a growing need to tailor immunoglobulin replacement therapy for individual patients and disease characteristics.
In this context, we analyzed clinical data from the Primary Immunodeficiency Database in Japan ver.2 (PIDJ2) to investigate the status of immunoglobulin replacement therapy in Japan.
As of January 2026, 367 patients receiving immunoglobulin replacement therapy were registered in PIDJ2. The mean age was 27 years (ranging from 0-80 years), with 209 males and 158 females. By disease category, antibody production deficiency accounted for approximately 60% of cases, followed by combined immunodeficiency and immune dysregulation disorders. It was also used in phagocytic disorders, innate immune disorders, and autoinflammatory diseases.
Intravenous immunoglobulin (IVIG) was administered to 136 patients, while subcutaneous immunoglobulin (SCIG) was used in 181 patients; among these, 50 transitioned from IVIG to SCIG during the observation period.
The mean IgG level was higher in patients receiving SCIG (981 mg/dL) than in those receiving IVIG (806 mg/dL). IgG levels at the onset of infectious events were available for 107 events. The mean IgG level during infections requiring hospitalization was 546 mg/dL, whereas it was 760 mg/dL in infections manageable in the outpatient setting. These findings suggest that maintaining higher IgG levels may be beneficial for patients at increased risk of severe infection or hospitalization.
