A 20-year-old female presents with a 5-day history of fatigue, lowgrade fever, and sore throat. Physical examination reveals bilateral enlarged, tender cervical lymph nodes, an exudative tonsillitis, and an enlarged spleen. A complete blood cell count reveals the hemoglobin and platelet counts to be within normal limits. The total white blood cell count is increased to 9200 cells per uL. Examination of the peripheral blood reveals the presence of atypical mononuclear cells with abundant cytoplasm. These cells have peripheral condensation of the cytoplasm, which gives them a "ballerina ski" appearance. Which one of the listed findings is most likely to be present in this individual?