Lymphocytes are produced by lymphoid organs and are a class of cells that produce and transport antibodies and defend against viral infections, and are an important cellular component of the body’s immune response function. There are five types of leukocytes, namely neutrophils, eosinophils, basophils, lymphocytes and monocytes. The percentage of lymphocytes is the percentage of cells of that form in the total white blood cell count. The lymphocyte percentage in normal physiological conditions is: 20%-40%. Decreased lymphocyte percentage: mainly seen with the application of adrenocorticosteroids, alkylating agents, anti-lymphocyte globulin and other treatments, as well as radiation damage, immunodeficiency diseases, and gammaglobulin deficiency. The absolute number of lymphocytes is reduced and the percentage is decreased. In some other cases, the number of other cells increases, such as leukemia and severe bacterial infections, and the number of neutrophils increases dramatically and the percentage of lymphocytes decreases relatively. Increased percentage of lymphocytes: mainly seen in infectious diseases and mainly viral infections, such as: measles, chickenpox, mumps, viral hepatitis, epidemic hemorrhagic fever, but also in whooping cough, tuberculosis, brucellosis, syphilis, etc. The number of lymphocytes increases in these cases, as well as their percentage. In addition, lymphocytosis can occur in neoplastic diseases (leukemia, lymphoma), during recovery from acute infectious diseases, and after organ transplantation. There are also conditions that cause a decrease in the number of other cells and a relative increase in the percentage of lymphocytes, such as abnormal hematopoietic function. Therefore, there are many reasons for changes in lymphocyte percentage, and it is not meaningful to diagnose diseases based on low lymphocyte percentage in routine blood alone; it can only be used as an initial screening test, and needs to be combined with clinical symptoms and other relevant laboratory tests.