Circulating immune complexes and complement sequence activation in infectious mononucleosis.

The role of immune complex formation was investigated in a patient with infectious mononucleosis complicated by an urticarial rash. Circulating cryoprotein immune complexes were identified during the urticarial phase of the illness, and disappeared during recovery. These complexes were composed of immunoglobulins G (IgG), M (IgM) and A (IgA), complement components C3, C4 and C5, Epstein-Barr (EB) virus capsid antibody and particles resembling EB virus. The IgG subtypes identified in the immune complexes were the complement fixing IgG-1, IgG-2 and IgG-3. The C3 activator of the properdin complex was detected in serum obtained during that acute phase but not after recovery. Thus, the transient appearance of circulating complement-fixing immune complexes was associated in this patient with activation of both classic and alternate complement pathways. The findings suggest that these complexes may be involved in the rash associated with infectious mononucleosis.