Decreased lymphocyte count refers to a lower than normal number of lymphocytes in the human peripheral blood system, which is commonly seen in acute bacterial infections, long-term use of drugs such as glucocorticosteroids, hematopoietic failure, and complications from radiotherapy and chemotherapy. Lymphocytes are mainly involved in human immunity, such as B-lymphocytes can differentiate themselves into plasma cells and then secrete antibodies; while T-lymphocytes are mainly involved in cellular immunity. 1. When patients have acute bacterial infections, especially acute purulent bacterial infections, the neutrophil count of peripheral blood will be abnormally high, which will further cause the lymphocyte count to decrease. 2. Patients with a history of prolonged glucocorticoid therapy, which can further reduce the lymphocyte count due to the inhibitory effect of glucocorticoids on lymphocyte production. 3. Patients with severe myelopoietic failure (e.g., acute aplastic anemia) will have lower lymphocyte counts due to myelopoietic failure and a drastic decrease in the number of whole blood cells. 4. After high-dose radiotherapy and chemotherapy, all kinds of hematopoietic stem cells in the body have been severely damaged, and the number of lymphocytes in the body will be reduced. The reason why patients have low lymphocyte counts needs further examination to be determined. It is recommended that patients go to regular hospitals for examination and treatment, eat and drink rationally, have a regular routine and improve the body’s immunity. The examination and treatment should be carried out under the guidance of a physician.