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The peripheral blood smear of a 5-year-old boy with infectious mononucleosis. Enlarged atypical lymphocytes are seen with abundant basophilic cytoplasm.
They atypical lymphocytes are comprised of a predominance of Downey type II cells (larger cells with abundant pale cytoplasm that often abut or partially surround adjacent red blood cells) and Downey type III cells (large cells with one or more prominent nucleoli and abundant deeply basophilic cytoplasm). The detection of Epstein–Barr virus (EBV)-associated Immunoglobulin M (IgM) antibodies using an indirect enzyme-linked immunosorbent assay (ELISA) confirmed the presence of an acute infection in this child.