There are three types of blood cells in the human body, namely, white blood cells, red blood cells and platelets. All three types of blood cells go through a primitive stage, an infantile stage, and a mature stage, which is the general process of blood cell production. Leukocytes begin at a stage called primitive leukocytes, then go through early juvenile leukocytes, intermediate juvenile leukocytes, and late juvenile leukocytes before finally developing and maturing into mature leukocytes. Erythrocytes also go through primitive erythrocytes, early erythrocytes, intermediate erythrocytes, late erythrocytes, reticulocytes, and finally mature erythrocytes. The predecessor cells of platelets are megakaryocytes, and megakaryocytes also go through primitive megakaryocytes, naive megakaryocytes, granulocytes, and mature megakaryocytes before finally being able to divide to produce platelets. All blood cells go through stages from infantile to mature, and infantile blood cells rarely appear in peripheral blood. If there is an excessive number of infantile cells in peripheral blood, it is often a pathological disease, most commonly leukemia.