The organs that produce leukocytes vary from time to time. In adults, leukocytes are mainly produced through the bone marrow, lymph nodes, liver, spleen, and other organs. Leukocytes mainly include neutrophils, eosinophils, basophils, lymphocytes and monocytes. As for the human body development process, the hematopoietic parts are different in different periods. The hematopoietic organs in embryonic period are yolk sac, liver and spleen, and the bone marrow, thymus and lymph nodes produce blood in fetal period. In adulthood, the thymus begins to atrophy, the lymph nodes produce lymphocytes and plasma cells, and the bone marrow becomes the main force for producing white blood cells. When bone marrow hematopoiesis is inadequate, extramedullary hematopoietic organs, including the spleen and liver, can be activated to produce leukocytes. White blood cells are important immune cells in the human body. If the test is abnormal, it is recommended to seek medical attention and standardized treatment.