TYPES OF WHITE BLOOD CELLS
White blood cells, or leukocytes, function primarily to control various disease conditions. Although these cells do most of their work outside the circulatory system, they use the blood for transportation.

Normally, five types of white cells can be found in the circulation blood. They are distinguished by their size, the nature of their cytoplasm, the shape of their nucleus, and their staining characteristics. For example, some types of leukocytes have granular cytoplasm and make up a group called granulocytes, while others lack cytoplasmic granules and are called agranulocytes.

Neutrophils are characterized by the presence of fine cytoplasmic granules that stain pinkish in neutral stain. The nucleus of a neutrophil is lobed and consists of two to five parts connected by thin strands of chromatin. Neutrophils account for 54 percent to 62 percent of the white cells in a normal blood sample.

Eosinophils contain coarse, uniformly sized cytoplasmic granules that stain deep red in acid stain. The nucleus usually has only two lobes. These cells make up 1 percent to 3 percent of the total number of circulating leukocytes.

Basophils are similar to eosinophils in size and in the shape of their nuclei, but they have fewer, more irregularly shaped cytoplasmic granules that stain deep blue in basic stain. This type of leukocyte usually accounts for less than 1 percent of the white cells.

The leukocytes of agranulocyte group include monocytes and lymphocytes. Both of these arise from red bone marrow; however, lympocytes also are formed in the organs of the lymphatic system.

Monocytes are the largest cells found in the blood, having diameters two to three times greater than those of red cells. Their nuclei vary in shape and are described as round, kidney-shaped, oval, or lobed. They usually make up 3 percent to 9 percent of the leukocytes in a blood sample and seem to live for several weeks for even months.

Although large lymphocytes are sometimes found in the blood, usually they are only slightly larger than the red cells. A typical lymphocyte contains a relatively large, round nucleus surrounded by a thin rim of cytoplasm. These cells account for 25 percent to 33 percent of circulating white blood cells. They seem to have relatively long life spans, which may extend for years.

WHITE BLOOD CELL COUNTS
The number of white blood cells in a cubic millimeter of human blood, called the white blood cell count (WBC), normally varies from 5,000 to 10,000. Because this number may change in response to abnormal conditions, white blood cell counts are clinical interest. For example, some infectious diseases are accompanied by a rise in the number of circulating white cells, and if the total number of white cells exceeds 10,000 per mm3 of blood, the person is said to have leukocytosis. This condition occurs during certain acute infections, such as appendicitis.

If the total white cell count drops below 5,000 per mm3 of blood, the condition is called leukopenia. Such a deficiency may accompany typhoid. influenza, measles, mumps, chicken pox, and poliomyelitis.

A differential white blood cell count (DIFF) is one in which the percentages of the various types of leukocytes in a blood sample are determined. This test is useful because the relative proportions of white cells may change in particular diseases. Neutrophils, for instance, usually increase during bacterial infections, and eosinophils may increase during certain parasitic infections and allergic reactions.

FUNCTIONS OF WHITE BLOOD CELLS
As mentioned previously, white blood cells function to control disease conditions, such as infections caused by microorganisms. For example, some leukocytes phagocytize microorganisms that get into the body, and other leukocytes produce substances (antibodies) that react to destroy or disable such microorganisms.

Leukocytes can squeeze between the cells that form blood vessel walls. This movement, which is called diapedesis, allows the white cells to leave the circulation. Once outside the blood, they move through interstitial spaces with an amoebalike self-propulsion, called amoeboid motion.

The most mobile and active phagocytic leukocytes are neutrophils and monocytes. Although neutrophils are unable to ingest particles much larger than bacterial cells, monocytes can engulf relatively large objects. Both of these phagocytes contain numerous lysosomes, which are filled with digestive enzymes capable of breaking down various proteins and lipids, such as those in bacterial cell membranes. As a result of their activities, neutrophils and monocytes often become so engorged with digestive products and bacterial toxins that they also die.

Eosinophils are only weakly phagocytic. However, they are attracted when various parasites enter tissues, and they can kill some of these invaders. Eosinophils also help to control allergic reactions by removing certain substances involved in such reactions.

Some of the cytoplasmic granules of hasophils contain a blood-clot inhibiting substance, called heparin, and other granules contain histamine. It is thought that basophils may aid in preventing intravascular blood clot formation by releasing heparin, and that they may cause an increase in blood flow to injured tissues by releasing histamine. Basophils are also involved in certain allergic reactions.

Lymphocytes play an important role in the process of immunity. Some, for example, form antibodies that act against specific foreign substances when they enter the body.