Leukocyte count may increase when the bone marrow production increases or when the exit of WBCs from the blood is decreased. In adults, the normal leukocyte count is x109/L leukocytes. Leukocytosis occurs when the count becomes 15,000-20,000 per microliter. Leukocytosis is a response to noxious stimulus that is part of an inflammatory reaction; it is usually accompanied by certain cytologic abnormalities like Dohle bodies or toxic granulation. Leukocyte count greater than 25 to 30 x 109/L is called as leukemoid reaction, exhibited by a healthy bone marrow when subjected to extreme trauma, stress and infection.
This increase in leukocyte (primarily neutrophils) is usually accompanied by a "left upper shift" in the ratio of immature to mature neutrophils and macrophages. The proportion of immature leukocytes decreases due to proliferation and inhibition of granulocyte and monocyte precursors in the bone marrow which is stimulated by several products of inflammation including C3a and G-CSF. Although it may indicate illness, leukocytosis is considered a laboratory finding instead of a separate disease . This classification is similar to that of fever , which is also a test result instead of a disease. [ citation needed ] "Right shift" in the ratio of immature to mature neutrophils is considered with reduced count or lack of "young neutrophils" (metamyelocytes, and band neutrophils ) in blood smear , associated with the presence of "giant neutrophils". This fact shows suppression of bone marrow activity, as a hematological sign specific for pernicious anemia and radiation sickness .