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A 36-year-old male presented with a 7-day history of moderate to high-grade fever and sore throat. Physical examination revealed mild tachypnea, tachycardia, and multiple freely mobile, non-tender cervical lymph nodes (around 1 cm in size). Additionally, a palpable liver of 2-3 cm, firm and non-tender, was noted on abdominal examination.

Hematological investigations showed Hemoglobin (Hb) at 11.5g/dl, Total Leukocyte Count (TLC) at 9900/cmm, and a differential white blood cell count indicating Neutrophils 15%, Lymphocytes 77%, Eosinophils 0%, Monocytes 6%, and Band cells 2%. The Platelet Count was 2,63,000/cmm. Biochemical investigations revealed elevated SGPT (525 IU/L), SGOT (218 IU/L), ALP (293 IU/L), and LDH (386 IU/L).

Peripheral smear examination demonstrated lymphocytosis with atypical lymphocytes, recognized as Downey cells type 1,2 and 3 (Figure-A, B and C respectively). These findings are often associated with various conditions, including Infectious Mononucleosis (IM) due to Epstein-Barr virus (EBV) infection, as well as other infectious and non-infectious causes.

Downey type1 cells are smaller in size, have indented to lobulated nucleus and cytoplasmic granules. Type 2 cells are of most common type and have abundant agranular cytoplasm, darker at the periphery, moulds around RBCs and radiating basophilia. Type 3 cells are balstoid in appearance with fine to coarse chromatin, nucleoli and deeply basophilic cytoplasm.

The diagnosis of IM is based on clinical examination, featuring the classic triad of fever, lymphadenopathy, and pharyngitis, along with laboratory findings such as atypical lymphocytosis, heterophile antibodies, and EBV-specific antibodies (anti-VCA, anti-EBNA, and anti-EA). The atypical lymphocytes, or Downey cells, are larger CD8+ cytotoxic T cells adherent around erythrocytes, supporting the diagnosis.

Possible differential diagnoses include other infectious causes such as TORCH infections, hepatitis B, syphilis, HIV, and SARS-CoV-2, as well as non-infectious conditions like autoimmune disorders, Hodgkin’s disease, sarcoidosis, drug-induced changes, and immune reactions.

In conclusion, the patient's clinical presentation and laboratory findings including atypical lymphocytosis, strongly suggest a diagnosis of Infectious Mononucleosis.

1. Simon, MW. The atypical lymphocyte. Int Pediatr 2003;18:20–2.

 2. van der Meer, W, van Gelder, W, de Keijzer, R, Willems, H. The divergent morphological classification of variant lymphocytes in blood smears. J Clin Pathol 2007;60:838–9