Lymphocytosis is an increase in the content of lymphocytes above 4000 (or 40%) in 1 µl of blood. The cause is acute viral diseases, some chronic bacterial infections, malignant hematological diseases, etc. Clinical manifestations are determined by the nosology against which lymphocytosis developed. The level of lymphocytes is examined in venous or capillary blood when calculating the leukocyte formula of the general blood test. To correct lymphocytosis, a fight against the cause is carried out, i.e. treatment of the underlying disease.