A 23-year-old male presents with fever and cervical lymphadenopathy. The peripheral smear shows the following finding. Diagnosis is

Infectious mononucleosis *Causative agent: Ebstein Barr virus (HHV-4)*Mode of transmission: Direct contact with human secretions, most often by saliva *Infected cells: Epithelial cells and B cells in the oropharynx*EBV infects B cells through C3d complement receptor (CD21) for which the co-factor used is MHC-II.*By age of 3 years, about 100% of children have positive serological findings for EBV. But not all those infected individuals acquire the disease.*EBV replicates in B cells, inducing expression of viral capsid antigen (VCA), membrane antigen (MA), early antigen (EA), EBV nuclear antigens (EBNA), and lymphocyte-detected membrane antigen (LYDMA).*Antibodies formed against these antigens were called heterophil antibodies which play a role in containing the infection*Incubation period: 2 to 5 weeks *Triad of symptoms: Fever, pharyngitis, and cervical /generalized lymphadenopathy*Peripheral blood: Shows atypical lymphocytes which are T cell type (however EBV affects B cells) with abundant pale blue cytoplasm, large nuclei and conspicuous nucleoli called Downey cells *Paul bunnel test: Agglutination of sheep erythrocytes by patient's serum. Beef erythrocytes (specific for infectious mononucleosis ) and guinea pig kidney are also used. *Monospot test: Most sensitive test in which agglutination was formed between horse erythrocytes and patient's serum.*Serology: To detect EBV specific antibodies VCA (IgM), VCA (IgG), EA and EBNA.Ref: Iochim's lymph node pathology p78