Abnormal lymphocytes are lymphocytes with abnormal and atypical morphology, and there are three clinical types: foamy, irregular, and naive. Abnormal lymphocytes can also be seen in the peripheral blood of normal individuals, but in small numbers, no more than 2%. If abnormal lymphocytes are significantly increased, the most common cause is an infectious disease, especially in the case of infectious mononucleosis, where the abnormal lymphocytes often exceed 10%, which is one of the conditions for the diagnosis of infectious mononucleosis. In addition, epidemic hemorrhagic fever can also present with an increase in abnormal lymphocytes. Certain bacterial infections, leptospirosis, rickettsial infections, and malaria can also present with elevated abnormal lymphocytes. In addition, drug allergies, such as penicillin allergy, blood transfusions and after hemodialysis, as well as granulocyte deficiency and radiation therapy, may also present with abnormal lymphocytosis.