Monocyte is a type of white blood cell and is one of the important components of the body’s defense mechanism. It is derived from bone marrow hematopoietic stem cells and has functions such as phagocytosis, clearance and defense. Physiological increase is common in infants and children, while pathological increase is often associated with some infectious diseases and hematological diseases. Infectious diseases such as endocarditis, acute infections, typhoid fever, etc. Since monocytes are immature when they enter the blood from bone marrow, they can enter the tissues after staying in the blood for 10-20 hours and continue to develop into macrophages, which have stronger phagocytic ability at this time and can engulf more and larger bacteria and particles, while the esterases in the lysosomes of macrophages can digest the lipid membranes of bacteria such as Mycobacterium tuberculosis and achieve better defense effect. Therefore, when a large number of bacteria invade, monocytes immediately enter the bloodstream to exert anti-infective effects, and the absolute monocyte value rises. Infectious mononucleosis is an acute, self-limiting disease caused by infection with human herpesvirus type 4 (EBV). In addition, acute mononuclear granulocytic leukemia and lymphoma are also seen as mononuclear cell increases. It is important to note that in the case of recovery from an infectious disease, the monocytes in the body need to continue to clear the body of residual material to avoid a relapse of the disease. Therefore, the test may show an increase in absolute monocyte values. In addition, a high absolute blood mononuclear cell count is not the only criterion to confirm the diagnosis of the disease. Patients need to improve the examination and determine whether they are suffering from the disease according to the clinical symptoms, and at the same time, actively treat the disease under the guidance of the doctor for a good prognosis.