he term leukocytosis refers to an increased concentration of circulating white blood cells. This parameter integrates the effects of differential cell counts and is dependent on both physiological (age, diurnal rhythm, pregnancy, physical exercise, food intake, stress) and pathological changes. Total and differential leukocyte counts are the most frequently used laboratory tests to discriminate between inflammatory and other events 1 . In the adult, any peripheral white blood cell count of ΟΎ10 000 mm Οͺ3 indicates leukocytosis (see Table 1 for normal white blood cell counts). Most commonly, this is the result of an increased number of neutrophils (neutrophilia), and is less often due to lymphocytosis and rarely to monocytosis, eosinophilia or basophilia. When leukocyte numbers increase to approximately 100 000 cells mm Οͺ3 , the term (myeloid or lymphatic) leukemoid reaction is used. In leukemoid reactions, preleukemias and leukemias, a simple blood smear analysis often leads to the diagnosis.
It is accepted that homeostatic mechanisms of production, release and elimination keep the levels of each individual peripheral blood cell type within limits, and that an increased blood cell count is central in the replenishment of white blood cells to the periphery. For specific subpopulations, for instance T and B cells, there are emerging data on factors that contribute to cell population dynamics 2,3 (Box 1). Because of the limited lifespan (days) and proliferative capacity of most phagocytes, the phenomenon of leukocytosis is essential in the primary host defense against infections. However, these kinetics are completely different for subsets of lymphocytes, which may contribute little to the leukocytosis but often have a long lifespan (e.g. years for memory cells), and may proliferate extensively in the periphery. To simplify the picture, this article will mainly deal with leukocytopoiesis of phagocytes in the bone marrow and phagocyte circulation. Furthermore, the discussion will take a reductionist approach regarding the vast differences in venous (and arterial) blood cell counts that are dependent on the anatomical site of sampling and the local environment. Although such differences might have an effect on differential white blood cell counts and on leukocytosis, an analysis of these additional complexities is beyond the scope of this article.