Leukocytes, or white blood cells, are nucleated blood cells that lack hemoglobin. They are clear or whitish in color and are larger than erythrocytes, ranging from 8 to 19 mm in diameter. In stained preparations leukocytes attract stain, whereas erythrocytes remain relatively unstained.
Leukocytes protect the body against invading microorganisms and remove dead cells and debris from the body. Leukocytes leave the circulation by the process of diapedesis (movement through vessel walls) and move through the tissues in which they ingest foreign material or dead cells. At the site of an infection leukocytes accumulate and phagocytize bacteria, dirt, and dead cells; then they die. This accumulation of dead leukocytes, along with fluid and cell debris, is called pus.
Leukocytes are named according to their appearance in stained preparations. Leukocytes containing large cytoplasmic granules are granulocytes, and those with very small granules that cannot be seen easily with the light microscope are agranulocytes. The three types of granulocytes are named according to the staining characteristics of their cytoplasm: neutrophils, eosinophils, and basophils. There are two types of agranulocytes: monocytes and lymphocytes.
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