Increased serum IgG4 levels in acute Epstein-Barr viral mononucleosis

Changes in serum IgG4 levels during heterophil-positive infectious mononucleosis were recorded. Pre-illness serial acute and convalescent specimens were evaluated. Mean IgG4 levels increased by 75% during acute infectious mononucleosis, reached a peak 21/2 to 3 weeks post-onset, and declined to baseline by 60 days. IgG4 peaked independently of other immunoglobulin classes.