1.an abnormal increase in the number of white blood cells in the blood as a result of infection (as in leukemia)
1.(MeSH)A transient increase in the number of leukocytes in a body fluid.
definition of Wikipedia
Blood Diseases, Hematological Diseases, Hematologic Dis, Hematologic Diseases - Pathological Conditions, Signs and Symptoms, Symptoms and General Pathology - Blood Cell Count, Blood Cell Number, Blood Count, Complete[Hyper.]
Leukocytosis (n.) [MeSH]
blood disease, blood disorder[Hyper.]
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|Classification and external resources|
Leukocytosis is a white blood cell count (the leukocyte count) above the normal range in the blood. It is frequently a sign of an inflammatory response, most commonly the result of infection, and is observed in certain parasitic infections. It may also occur after strenuous exercise, convulsions such as epilepsy, emotional stress, pregnancy and labour, anesthesia, and epinephrine administration.
There are five principle types of leukocytosis:
Acute exercise is one of the healthiest ways to create leukocytosis within hours. This increase in leukocytes (primarily neutrophils) is usually accompanied by a "left shift" in the ratio of immature to mature neutrophils. The increase in immature leukocytes increases due to proliferation and release 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. "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. 
A leukocyte count above 25 to 30 x 109/L is termed a leukemoid reaction, which is the reaction of a healthy bone marrow to extreme stress, trauma, or infection. It is different from leukemia and from leukoerythroblastosis, in which either immature white blood cells (acute leukemia) or mature, yet non-functional, white blood cells (chronic leukemia) are present in peripheral blood.
Below are blood reference ranges for various types leucocytes/WBCs. The 97.5 percentile (right limits in intervals in image, showing 95% prediction intervals) is a common limit for defining leukocytosis.
Leukocytosis can be subcategorized by the type of white blood cell that is increased in number. Leukocytosis in which neutrophils are elevated is neutrophilia; leukocytosis in which lymphocyte count is elevated is lymphocytosis; leukocytosis in which monocyte count is elevated is monocytosis; and leukocytosis in which eosinophil count is elevated is eosinophilia.
|Causes of leukocytosis|
Leukocytosis is very common in acutely ill patients. It occurs in response to a wide variety of conditions, including viral, bacterial, fungal, or parasitic infection, cancer, hemorrhage, and exposure to certain medications or chemicals including steroids.
For lung diseases such as pneumonia and tuberculosis, WBC count is very important for the diagnosis of the disease, as leukocytosis is usually present.
The mechanism that causes leukocytosis can be of several forms: an increased release of leukocytes from bone marrow storage pools, decreased margination of leukocytes onto vessel walls, decreased extravasation of leukocytes from the vessels into tissues, or an increase in number of precursor cells in the marrow.
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