Lymphocytes are very important immune cells in the human body, which are responsible for tasks such as fighting viruses with specific pathogens, secreting the body’s immunoglobulins, and recognizing the role of self and non-self. Lymphocyte reduction is mainly seen in the following conditions: 1, immune deficiency diseases, especially congenital gammaglobulin deficiency, this type of disease is congenital, manifested by the insufficient number of lymphocyte production. 2, too much destruction, mainly seen in the use of adrenocorticotropic hormones, alkylating agents, anti-thymocyte immunoglobulin and radiation, can damage and destroy a large number of lymphocytes. 3 In addition, the number of lymphocytes in the body is significantly reduced, and in this case the body becomes immunocompromised, and the patient is prone to an increase in opportunistic infections, most commonly known as AIDS. In such patients, the number of CD4 and T helper lymphocytes in the body is significantly reduced due to retroviral infection, and then the patient is prone to secondary infections including cytomegalovirus and fungal infections, which can be life-threatening in severe cases.