White blood cells (leukocytes), unlike red cells, are nucleated and independently motile. Highly differentiated for their specialized functions, they do not undergo cell division (mitosis) in the bloodstream, but some retain the capability of mitosis. As a group they are involved in the body’s defense mechanisms and reparative activity. The number of white cells in normal blood ranges between 4,500 and 11,000 per cubic millimetre. Fluctuations occur during the day; lower values are obtained during rest and higher values during exercise. Intense physical exertion may cause the count to exceed 20,000 per cubic millimetre. Most of the white cells are outside the circulation, and the few in the bloodstream are in transit from one site to another. As living cells, their survival depends on their continuous production of energy. The chemical pathways utilized are more complex than those of the red cells and are similar to those of other tissue cells. White cells, containing a nucleus and able to produce ribonucleic acid (RNA), can synthesize protein. They comprise three classes of cells, each unique as to structure and function, that are designated granulocytes, monocytes, and lymphocytes.

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Four methicillin-resistant Staphylococcus aureus (MRSA) bacteria (purple) being engulfed by neutrophils (blue), which are a type of human white blood cell.
National Institute of Allergy and Infectious Diseases/Centers for Disease Control and Prevention (CDC)

Granulocytes

Granulocytes, the most numerous of the white cells, are larger than red cells (approximately 12–15 μm in diameter). They have a multilobed nucleus and contain large numbers of cytoplasmic granules (i.e., granules in the cell substance outside the nucleus). Granulocytes are important mediators of the inflammatory response. There are three types of granulocytes: neutrophils, eosinophils, and basophils. Each type of granulocyte is identified by the colour of the granules when the cells are stained with a compound dye. The granules of the neutrophil are pink, those of the eosinophil are red, and those of the basophil are blue-black. About 50 to 80 percent of the white cells are neutrophils, while the eosinophils and basophils together constitute no more than 3 percent.

Neutrophils

The neutrophils are fairly uniform in size with a diameter between 12 and 15 μm. The nucleus consists of two to five lobes joined together by hairlike filaments. Neutrophils move with amoeboid motion. They extend long projections called pseudopodium into which their granules flow; this action is followed by contraction of filaments based in the cytoplasm, which draws the nucleus and rear of the cell forward. In this way neutrophils rapidly advance along a surface. The bone marrow of a normal adult produces about 100 billion neutrophils daily. It takes about one week to form a mature neutrophil from a precursor cell in the marrow; yet, once in the blood, the mature cells live only a few hours or perhaps a little longer after migrating to the tissues. To guard against rapid depletion of the short-lived neutrophils (for example, during infection), the bone marrow holds a large number of them in reserve to be mobilized in response to inflammation or infection. Within the body, the neutrophils migrate to areas of infection or tissue injury. The force of attraction that determines the direction in which neutrophils will move is known as chemotaxis and is attributed to substances liberated at sites of tissue damage. Of the 100 billion neutrophils circulating outside the bone marrow, half are in the tissues and half are in the blood vessels. Of those in the blood vessels, half are within the mainstream of rapidly circulating blood, and the other half move slowly along the inner walls of the blood vessels (marginal pool), ready to enter tissues on receiving a chemotactic signal from them.

Neutrophils are actively phagocytic; they engulf bacteria and other microorganisms and microscopic particles. The granules of the neutrophil are microscopic packets of potent enzymes capable of digesting many types of cellular materials. When a bacterium is engulfed by a neutrophil, it is encased in a vacuole lined by the invaginated membrane. The granules discharge their contents into the vacuole containing the organism. As this occurs, the granules of the neutrophil are depleted (degranulation). A metabolic process within the granules produces hydrogen peroxide and a highly active form of oxygen (superoxide), which destroy the ingested bacteria. Final digestion of the invading organism is accomplished by enzymes.

Eosinophils

Eosinophils, like other granulocytes, are produced in the bone marrow until they are released into the circulation. Although about the same size as neutrophils, the eosinophil contains larger granules, and the chromatin is generally concentrated in only two nonsegmented lobes. Eosinophils leave the circulation within hours of release from the marrow and migrate into the tissues (usually those of the skin, lung, and respiratory tract) through the lymphatic channels. Like neutrophils, eosinophils respond to chemotactic signals released at the site of cell destruction. They are actively motile and phagocytic. Eosinophils are involved in defense against parasites, and they participate in hypersensitivity and inflammatory reactions, primarily by dampening their destructive effects.

Basophils

Basophils are the least numerous of the granulocytes, and their large granules almost completely obscure the underlying double-lobed nucleus. Within hours of their release from the bone marrow, basophils migrate from the circulation to the barrier tissues (e.g., the skin and mucosa), where they synthesize and store histamine, a natural modulator of the inflammatory response. When aggravated, basophils release, along with histamine and other substances, leukotrienes, which cause bronchoconstriction during anaphylaxis (a hypersensitivity reaction). Basophils incite immediate hypersensitivity reactions in association with platelets, macrophages, and neutrophils.

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Monocytes

Monocytes are the largest cells of the blood (averaging 15–18 μm in diameter), and they make up about 7 percent of the leukocytes. The nucleus is relatively big and tends to be indented or folded rather than multilobed. The cytoplasm contains large numbers of fine granules, which often appear to be more numerous near the cell membrane. Monocytes are actively motile and phagocytic. They are capable of ingesting infectious agents as well as red cells and other large particles, but they cannot replace the function of the neutrophils in the removal and destruction of bacteria. Monocytes usually enter areas of inflamed tissue later than the granulocytes. Often they are found at sites of chronic infections.