The complement system in infectious mononucleosis.

The complement system was investigated in 34 patients with infectious mononucleosis. Three had specific complications: one haemolytic anaemia, one severe arthralgia/myalgia and one proliferative glomerulonephritis. Complement changes consistent with classical pathway consumption were seen in ten of the uncomplicated group and the patients with haemolytic anaemia and arthralgia/myalgia. The patient with glomerulonephritis showed evidence of alternative pathway utilisation including C3 splitting activity and the deposition of properdin on renal biopsy. The complement findings suggest that circulating immune complexes are common in such patients and are likely to play a role in the pathogenesis of the complications. It is proposed that both complement pathways may be required for the effective clearance of viral material from the circulation.