OBJECTIVETo study the clinical and morphological features, immunophenotype and in situ detection of Epstein-Barr virus (EBV) infection in infectious mononucleosis (IM) to enhance the knowledge and diagnosis of the disease.

METHODUsing routine haematoxylin and eosin staining, immunohistochemistry and EBER in situ hyhridization together with clinical data analysis, 15 cases of IM were evaluated for their clinical features, morphology, immunophenotype and EBV infection status.

RESULTSIM was common in children and young adults with a median age of 18 years. It was an acute disease with lymphadenopathy and frequently fever. Most of the patients had a rapid recovery. Every case showed a markedly T zone expansion with a mottling pattern, composing of small to large lymphocytes, plasma cells and histiocytes. The cells also showed a B-cell differentiation profile ranging from activated lymphoblastoid cells, immunoblasts, plasmablasts, plasma-like cells and plasma cells. Many small lymphocytes in the expanded T zone expressed CD3. Some of the activated lymphoblastoid cells and immonoblasts were CD20 and CD30 positive with variable intensity signals. EBER positive (nuclear staining) cells were seen in every case. The number of EBER positive cells ranged from 10 to more than 100 per high power field. These cells included small to large lymphocytes locating mostly in the expanded T zone and a few were in the follicular germinal centers.

CONCLUSIONSIM is an EBV related acute sell-recovering lymphoproliferative disease, having distinct clinical, morphological and immunophenotypic characteristics as well as EBV infection. Taking these features into consideration will facilitate the correct diagnosis of IM.