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 (oxygen and carbon_dioxide). 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.

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 granules (red, blue, or both) 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.

Granulocytes:

Neutrophils are the most prevalent WBC in the bloodstream. These cells are often referred to as "poly's" because they have a multilobed nucleus. They are granulocytes, and both colors of granules (red and blue) may be present in their cytoplasm. These cells are good bacterial phagocytes.

Eosinophils have a very distinctive appearance. They have a bilobed nucleus ("telephone-receiver" or "bar-bell" shaped) and many red-stained granules in their cytoplasm. These WBC's defend against parasitic worms and destroy antigen-antibody_complexes

Basophils have a very distinctive appearance. They have large numbers of dark-staining granules in their cytoplasm, and these granules appear to be "popping" right out of the cell. The nucleus of the basophil will appear "s-shaped", if it is visible at all. Often, the nucleus is concealed by the presence of numerous dark-staining granules. Basophils produce histamine and also contain heparin, which is an anticoagulant.

Agranulocytes:

(1) 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.