Downey type II

Downey cells are classified into three subtypes (I, II, III) based on morphological features, originally described by Hal Downey in the study of infectious mononucleosis. Type II Downey cells are large, with abundant foamy/vacuolated basophilic cytoplasm, eccentric nuclei, and cytoplasmic borders that appear to mold or 'hug' adjacent erythrocytes—a distinctive morphologic clue on smear review. These cells represent activated/reactive CD8+ T lymphocytes mounting a cytotoxic immune response against EBV-infected B cells displaying viral antigens. Pathophysiologically, EBV infects B cells via the CD21 receptor, leading to polyclonal B cell proliferation; the immune system responds with a robust T cell (especially CD8+) response, producing the atypical lymphocytosis characteristic of mononucleosis. Clinically, these atypical lymphocytes are a hallmark finding supporting the diagnosis of infectious mononucleosis, alongside fever, pharyngitis, lymphadenopathy, and splenomegaly. While Downey cells are most classically associated with EBV, similar atypical lymphocytes can be seen in other viral infections (CMV, HIV, toxoplasmosis, viral hepatitis) and are generally referred to as 'atypical lymphocytes' in modern hematology rather than strictly classified by Downey subtype. On boards, atypical lymphocytes with this morphology in a febrile young adult with lymphadenopathy should prompt consideration of a heterophile antibody test (Monospot) or EBV serologies for diagnosis confirmation. It's important to differentiate from leukemic blasts, as reactive atypical lymphocytes are polyclonal and benign, unlike the monoclonal proliferation seen in leukemias.