A high lymphocyte count refers to a higher than normal number of lymphocytes in the peripheral blood. In clinical practice, high lymphocyte count is commonly seen in: i. Viral infections, such as influenza virus, rubella virus, hepatitis B virus, etc. When these viral infections occur, they stimulate the body to have an immune response, which can lead to an elevated lymphocyte count. ii. Acute lymphoblastic leukemia, which is the most common type of leukemia in pediatric patients. It results in a significant increase in the number of lymphocytes in the peripheral blood due to the proliferation of immature leukemia cells in the bone marrow and their release into the peripheral blood. Third, chronic lymphocytic leukemia, which is common in older patients. The number of lymphocytes in the peripheral blood is increased due to the proliferation of seemingly mature lymphocytes in the bone marrow and their release into the peripheral blood, which appear to be mature but are in fact a defective cell.