Patients presenting with severe EBV infections should be focused on early identification of a possible primary immunuodeficiency or a chronic active EBV infection clinical condition (CAEBV) and haemophagocytic lymphohistiocytosis(HLH). Immunological phenotyping of NK-, T- and B-cell differentiation, and functional assays including cytotoxic cell killing function and cytotoxic granule release, provide a useful identification for clinical conditions inability to control EBV infections. Genomic DNA is isolated from peripheral blood mononuclear cells and will be amplified to screen for possible immunodeficiency.