Leukocytosis, defined as a white blood cell count greater than 11,000 per mm3 (11 X 109 per L), is frequently found in the course of routine laboratory testing. An elevated white blood cell count typically reflects the normal response of bone marrow to an infectious or inflammatory process. Occasionally, leukocytosis is the sign of a primary bone marrow abnormality in white blood cell production, maturation or death (apoptosis) related to a leukemia or myeloproliferative disorder. Often, the family physician can identify the cause of an elevated white blood cell count based on the findings of the history and physical examination coupled with basic data from the complete blood count.
Production, Maturation and Survival of Leukocytes Common progenitor cells, referred to as "stem cells," are located in the bone marrow and give rise to erythroblasts, myeloblasts and megakaryoblasts. Three quarters of the nucleated cells in the bone marrow are committed to the production of leukocytes. These stem cells proliferate and differentiate into granulocytes (neutrophils, eosinophils and basophils), monocytes and lymphocytes, which together comprise the absolute white blood cell count. An abnormal elevation in the neutrophil count (neutrophilia) occurs much more commonly than an increase in eosinophils or basophils.
|Table 1 -Pathophysiologic Mechanisms of Leukocytosis|
Normally responding bone marrow
Etiology of Leukocytosis
The investigation of leukocytosis begins with an understanding of its two basic causes:
1. the appropriate response of normal bone marrow to external stimuli and
2. the effect of a primary bone marrow disorder.
Physiologic mechanisms of leukocytosis are listed in Table 1.
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