Low lymphocytes include both absolute and relative decreases. Any of the various causes that lead to a significant increase in neutrophils can lead to a relative decrease in lymphocytes, where the lymphocyte ratio decreases, but the absolute number does not. Absolute decrease in lymphocytes is common in the following causes: recovery from influenza; early lymphocytopenia in tuberculosis, with a significant decrease in CD4+ cells, which can return to normal if treatment is effective; treatment with alkylating agents (cyclophosphamide, etc.) can cause a significant decrease in white blood cells with a significant decrease in lymphocytes, which can persist for several years after stopping treatment; radiation therapy can destroy lymphocytes. Each low-dose radiotherapy treatment produces more destruction than 2 high-dose radiotherapy treatments per week; immune diseases, such as systemic lupus erythematosus, rheumatoid arthritis, mixed connective tissue disease, and polymyositis, are reduced due to the production of anti-lymphocyte antibodies by antibodies, resulting in lymphocyte destruction, and the degree of reduction correlates with the titer of the antibodies; various types of congenital immunodeficiencies, such as severe combined immunodeficiency, Various types of congenital immunodeficiencies, such as severe combined immunodeficiency, ataxia capillaris, malnutrition, or zinc deficiency, can cause lymphopenia to varying degrees. Certain drugs can also cause lymphopenia, such as menadione, nitrogen mustard phenylbutyrate, cortisone, epinephrine, lithium, nicotinic acid, steroids, etc. Therefore, there are many causes of low lymphocytes, but low lymphocytes alone are not enough to make a definite diagnosis of the disease, and the specific cause needs to be combined with clinical symptoms and other relevant laboratory tests.