Leukocytes are a heterogeneous group of blood cells that all have nuclei. In the centrifugation of blood, they form a white layer between the erythrocytes and plasma called the leukocrit (buffy coat). This is rarely taken into consideration in normal leukocyte counts, but can be important in increased leukocyte counts, for example in leukemias (leukós = Greek for white).
Neutrophils, eosinophils, and basophilic granulocytes normally appear in blood as well as monocytes, lymphocytes and plasma cells. They are all involved in fighting infection and inflammation.
Myeloblasts, promyelocytes, myelocytes, metamyelocytes, leukemic blasts and smudge cells only occur pathologically in the peripheral blood. Normoblasts and megakaryocyte remnants also have nuclei and only occur pathologically in the peripheral blood. However, they are not classified as leukocytes. They are elements of erythropoiesis and/or of megakaryocytopoiesis.
The reference range of leukocytes is between and x 109/L. A decrease in leukocytes is known as leukopenia, and an increase as leukocytosis. Some ethnic groups (e. g. subsaharian Africans) physiologically have lower leukocyte counts.
Only absolute values count!
Because of heterogeneity, the leukocyte count is only a superficial parameter. The number of individual leukocyte populations is more important. Counting the leukocytes according to subpopulations is called a differential count. The results of a differential are given both in absolute numbers and in relative numbers (percent). The percentages are useless except in shift to the left. The absolute values are diagnostic! For example:
The normal range for eosinophils is to x 109/L or 0 to 5%. With a leukocyte count of 4.3 x to 109/L and an eosinophil count of 0.35 x 109/L, this percentage is 8%. This situation suggests no eosinophilia since the absolute eosinophil count is not increased. This relative eosinophilia would not need to be evaluated.