The number of leukocytes varies according to the age, sex, and physiologic conditions with normal adults having about 6,000-10,000 leukocytes per microliter of blood. Leukocytes are classified into granulocytes and agranulocytes based on the presence or absence of visible granules within the cellular cytoplasm. Granulocytes include neutrophils, eosinophils, and basophils. Agranulocytes are the monocytes and lymphocytes. Immature neutrophils have a nonsegmented horseshoe shaped nucleus.
Older neutrophils have more than five lobes and are called hypersegmented.
The cytoplasm contains small granules that are primary lysosomes containing numerous enzymes. There are few mitochondria in the cytoplasm and netrophils mostly survive by the anaerobic use of glycogen for energy. These phagocytic cells surround and engulf bacteria and constitute a defense against invasion. Eosinophils have a diameter of 12-15 microns and contains a characterisitic bilobed nucleus. The main identifying feature of eosinophils is the presence of many large and elongated granules that are stained red by eosin.