Key Points

• Leukocytosis is WBC count 2 SD above the mean, generally higher than 11,000/^L.

• Only 10% of all neutrophils are found in the peripheral blood, where they survive only 6 to 10 hours.

• Neutrophilia is caused by inflammation, stress, or steroid therapy.

• Infection is probable when band neutrophils are more than 20% of the total WBC count and Döhle bodies and toxic granulations are seen.

• The risk of severe infection does not usually occur unless the absolute neutrophil count is less than 500/^L.

The white blood cells (WBCs, leukocytes) are made up of neutrophils, lymphocytes, eosinophils, basophils, mono-cytes, and plasma cells, although plasma cells are rarely found in the circulation. Most are produced in the bone marrow and some primarily in lymphoid tissue. The normal WBC count ranges from approximately 4300 to 10,000 cells^L with some variation, depending on the laboratory reference control samples. An abnormal WBC count can be defined as more than 2 SD from the mean of 7000 cells^L.

Leukocytosis is defined as a WBC count 2 SD higher than the mean, generally greater than 11,000 cells^L. WBCs are composed of two types of cells with different functions. Phagocytic cells, consisting of neutrophils, eosinophils, monocytes, and macrophages, are primarily responsible for the ingestion and killing of microorganisms. Lymphocytes are responsible for cellular-mediated immunity and humoral immunity expressed through the production of antibodies. Table 39-4 lists normal WBC values and ranges. White cells produce cytokines (interleukins and growth factors), vasoactive substances, and enzymes, which allow them to interact and formulate immune responses. This section focuses primarily on phagocytic cells and their disorders; lymphocytes and lymphocytic disorders are discussed in a later section. When a patient has an increasing number or severity of bacterial or fungal infections, or both, abnormalities of phagocytic cells should be suspected. Conversely, when any phagocytic cell line is increased, such as neutrophils or eosinophils, an infectious process should be suspected.

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