Red blood cells are also called erythrocytes. White blood cells are also called leukocytes. Monocytes are the largest kind of leukocyte. Neutrophils engulf microorganisms by phagocytosis. Monocytes have a large, darkly-stained nucleus surrounded by a thin layer of clear cytoplasm. Erythrocytes are the most numerous formed element found in human blood. Basophils have a kidney shaped nucleus and gray-blue cytoplasm. Platelets lack a nucleus. Monocytes spend 2-3 days in the circulatory system, then move into the tissues. Eosinophils increase when humans are infected with parasitic worms.