Lymphocytes are a component of white blood cells, with values fluctuating between (0.8-4) x 10^9/L. Lymphocyte increase can be seen in both pathological and physiological conditions: physiological increase is most often seen in childhood, and the lymphocyte level gradually returns to normal adult levels by the age of 4-6 years; pathological increase is most often seen in viral infections, but also in acute and chronic lymphocytic leukemia, lymphoma and other malignant neoplastic diseases, as well as in the recovery from acute infectious diseases, and in transplant rejection. Lymphocytes can also increase during the time of transplant rejection. Therefore, it is necessary to correctly analyze the causes of lymphocytosis and provide symptomatic treatment in order to achieve the goal of treating the disease.