Formed elements in human peripheral blood include white blood cells (WBC's, leukocytes ), platelets (thrombocytes), and red blood cells (RBC's, erythrocytes ). These images show WBC's stained with Wright's stain at 1000X magnification. A normal adult would have 5000-10,000 WBC's per cubic millimeter of blood. WBC's are differentiated into granulocytes (have distinct cytoplasmic granules) and agranulocytes (lack cytoplasmic granules). The three major classes of polymorphonuclear granulocytes or PMN's are neutrophils , eosinophils , and basophils . The two classes of agranulocytes are monocytes and lymphocytes.

Neutrophils

Neutrophils, the most common WBC in peripheral blood, possess nuclei with two or more distinct lobes connected by thin filaments or bands of chromatin. Abundant small cytoplasmic granules may be present. These phagocytic cells are intermediate in size, ranging from 10 to 15 microns in diameter.

Eosinophils

Eosinophils are phogocytic cells containing a bilobed or banded nucleus. They are similar in size to neutrophils. The cytoplasm is lilac to blue, but is often difficult to see because of a large number of granules that stain a bright orange-red. Increased numbers of eosinophils are often present in patients with parasitic infections or allergies.

Basophils

Basophils, the rarest WBC in normal blood, are similar in size to neutrophils and eosinophils. Basophilic cytoplasm is lilac to blue, but is often difficult to see because of a large number of distinct blue granules.

Monocytes

Monocytes (the blood form of macrophages) are the largest WBC's normally found in blood (15 - 20 microns). The nucleus is frequently indented forming a horseshoe or "U" shape, or it may be folded. The nuclear structure is loose and stains a lighter purple than nuclei of other white cells. The cytoplasm is grey-blue.

Lymphocytes

The lymphocyte is a round or oval-shaped cell, ranging from small to intermediate in size (7 to 20 microns in diameter). It is the cornerstone of the immune system. The lymphocyte nucleus is usually round to slightly indented with a sharply defined edge,and deep, dense purple. Cytoplasm may be scant or form a narrow rim around the nucleus.