Common causes of high lymphocytes include viral infections, leukemia, and rejection reactions after organ transplantation. In addition, high lymphocytes can also occur in neonatal period and childhood. 1. Viral infections: such as viral cold, hepatitis B virus infection, chicken pox, etc. The virus will stimulate the body’s immune cells, thus causing high lymphocytes and low centrophils. 2. Leukemia: such as acute lymphoblastic leukemia, chronic lymphoblastic leukemia, etc.: due to the immature leukemia cells proliferating in large quantities in the bone marrow and releasing into the blood, resulting in high lymphocytes in the peripheral blood. Bone marrow aspiration etc. can be performed to confirm the diagnosis. 3. Rejection reaction after organ transplantation: such as bone marrow transplantation, liver transplantation, etc. When rejection reaction occurs, it will also cause high lymphocytes. 4. From 4~6 days after birth to 4~6 years old, high lymphocytes can occur, which is a normal phenomenon due to physiological reasons. In case of high lymphocytes, the cause of the disease should be clarified first. If it is caused by pathological reasons, treatment can be carried out under the guidance of physicians to avoid delaying the disease.