A lymphocyte is a type of white blood cell present in the blood. White blood cells help protect the body against diseases and fight infections. When the general defence systems of the body have been penetrated by dangerous invading microorganisms, lymphocytes help provide a specific response to attack the invading organisms. Lymphocytosis conventionally refers to a lymphocyte count greater than 5000/ml; however, a lymphocyte count exceeding this is physiologically present in infants and young children. Viral infection generally causes lymphocytosis (relative or absolute) with or without neutropaenia (low neutrophil count). It may be caused by acute viral infections (including hepatitis, infectious mononucleosis or cytomegalovirus infection, respiratory syncytial virus infections), chronic infections like tuberculosis and lymphoid malignancies (e.g. chronic lymphocytic leukaemia). The elevated lymphocyte count needs to be viewed in the light of the clinical profile. Please get a complete blood count with a peripheral blood film review. This would help decide if further investigations (like a bone marrow examination, lymph node biopsy etc.) are warranted.