Unlike erythrocytes that fulfil all their functions in the blood, leukocytes (granulocytes, lymphocytes, and monocytes) travel from the place where they have been produced to the place where they carry out their functions. That is why it is impossible to determine the life-span of a leukocyte in the peripheral blood. Leukocytes protect the organism from foreign substances by phagocytosis and production of antibodies. These two functions are closely linked to each other. Neutrophilic, eosinophilic, basophilic granulocytes, and monocytes perform phagocytosis. The production of antibodies is connected with lymphocytes and plasma-cells.
Phagocytes can be divided into two large groups:
- polymorphonuclear phagocytes (granulocytes), and
- mononuclear phagocytes (macrophages, monocytes, promonocytes, and their precursors in the bone marrow).
The number of leukocytes in the peripheral blood varies in wide limits (4-10 x 109/L) and is under the influence of different endogenous and exogenous factors. Simple physical activity is followed by an increase of leukocytes. Strenuous work may increase the number of leukocytes up to 22-35 x 109/L. This type of neutrophilia is a consequence of the immigration of neutrophilic granulocytes from marginal areas into the bloodstream. All short-lived physiological alterations in the number of leukocytes (mainly neutrophilic granulocytes) are the result of increased secretion of epinephrine releasing leukocytes from marginal areas. Similarly, increased secretion of cortisol may participate in producing physiological leukocytosis due to preventing neutrophilic granulocytes from moving into tissues out of blood, and increased transition of these leukocytes from the bone marrow to blood. |
