Blood consists of plasma (fluid) and formed elements ("cells"). Blood "cells" are called formed elements because only one of the three types of "cells" actually has complete cellular structure at maturity. The erythrocytes loose their nuclei and most of their organelles as they mature. They do so to provide sufficient space for hemoglobin molecules, which are responsible for transportation of respiratory gases. Erythrocytes are also known as RBC's or red blood cells. Platelets are cell fragments that lack a nucleus; therefore they are not "true cells" because they lack complete cellular structure at maturity. Platelets assist with the process of coagulation. Leukocytes retain their nuclei and organelles at maturity and are therefore "true" cells. Leukocytes are defensive cells and are also referred to as WBC's or white blood cells.
When viewing a stained prepared blood slide, RBC's will look small, anucleate and biconcave; platelets will appear as tiny fragments; and WBC's will vary in appearance, but will all have a well-developed nucleus.
WBC's can be classified based upon their variations in appearance. Two factors are involved: (1) the shape of the nucleus, and (2) the presence or absence of red or blue (or both) granules within the cytoplasm of the cell. Granules are actually cell organelles that react to chemicals used in the staining process.
There are two classes of WBC's: (1) agranulocytes, which lack distinctive granules in their cytoplasm, and (2) granulocytes, which contain red or blue (or both) staining granules in their cytoplasm.
Neutrophils, basophils, and eosinophils are granulocytes. Lymphocytes and monocytes are agranulocytes.