Red cell agglutinins are infrequently seen in infectious mononucleosis (∼1% of cases) and have been ascribed to polyclonal IgG/IgM cold agglutinins specific for the i antigen on the red cells. The weakly positive DAT using anti-C3d and the negative DAT using anti-IgG are characteristic for infectious mononucleosis, indicative of complement-mediated intravascular hemolysis that can occur 1 to 2 weeks after infection.
Red cell agglutination in infectious mononucleosis
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Source: Blood Journal