White blood cells or leukocytes are important cellular components of blood, and play a vital role in several immunity-related processes of the body. Under normal conditions, they comprise about 1% of the total blood volume, and their counts range between 4,500 and 10,000 per microliter of blood. The Complete Blood Count test consists of tests to evaluate the numbers of red blood cells (RBC), white blood cells (WBC) and platelets, measurement of hemoglobin, mean red cell volume, etc. It helps to evaluate the oxygen carrying capacity of blood, identify infections, as well as diagnose diseases (including cancers) that affect the bone marrow and production of blood cells. It is also useful to gauge the effect of chemotherapy and radiation therapy on blood production. WBCs are produced in the bone marrow as immature cells and released into the bloodstream. These immature cells then migrate to appropriate tissues, and differentiate into mature leukocytes. They are destroyed in the lymphatic system after 13 to 20 days from their synthesis in the bone marrow. The white blood cell count helps to provide information about various illnesses, and also helps monitor the patient's recovery once the treatment has started. The differential blood count reveals the absolute as well as relative numbers of each type of white blood cell, namely granulocytes (neutrophils, basophils, and eosinophils) and agranulocytes (lymphocytes, macrophages, monocytes). Newborns have a high WBC count which can range between 9000 to 30,000 cells per microliter of blood. Two weeks after birth, the numbers reduce to fall between the normal range of 4,500 to 10,000 per microliter of blood, with relative percentages of each cell type being as given below.

White Blood Cells	Percentage
Neutrophils	40 - 60
Lymphocytes	20 - 40
Monocytes	2 - 8
Eosinophils	1 - 4
Basophils	0.5 - 1

Leukocytosis can be the result of infection, inflammation or allergic reactions, and may reduce when the agent is removed. However, chronic infections as well as diseases, like blood cancer that leads to anomalous proliferation of white blood cells, may lead to persistently high WBC counts. White blood cells help in providing immunity against infections through phagocytosis of pathogens and pathogenic determinants, as well as by production of antibodies and messenger molecules. In case of acute infection, the white blood cells produce a substance known as the colony-stimulating factor (CSF), which stimulates the bone marrow to increase the white blood cell production. Production of the cells can be doubled within a few hours. This gives rise to fever, since a higher body temperature is required for optimal and fast production of WBCs. The precise cell types that are produced in increased quantities, depends on the specific infectious agent encountered by the immune system. Given below are a few examples: » Basophil: Chickenpox (varicella) and chronic sinusitis » Eosinophil: Scarlet fever, chorea, leprosy, genitourinary infections, parasitic infections (Löffler's syndrome), like ascariasis and trichinosis » Neutrophil: Purulent infections and formation of pyogenic abscesses » Lymphocytes: Acute infections like cytomegalovirus infection, Epstein-Barr virus infection, pertussis, hepatitis, and toxoplasmosis; Chronic infections like tuberculosis and brucellosis A high WBC count is the simplest indicator of an inflammatory response that may be triggered by tissue damage as well as allergies. It may occur due to infectious agents, chemical agents like drugs, pollen grains, etc. These conditions are characterized by increased migration of macrophages and granulocytes to the site of damage. The symptoms include redness, swelling, pain, and burning sensation at the site of inflammation, and rash or itching in case of skin allergies. » Basophil: Inflammatory bowel disease, chronic airway inflammation and chronic dermatitis » Eosinophil: Rheumatoid arthritis, periarteritis, and dermatological conditions » Neutrophil: Tissue injuries like lung injury during acute respiratory distress syndrome (ARDS) » Lymphocytes: Inflammatory bowel disease and vasculitis Leukocytosis is a characteristic feature of myeloproliferative disorders, that involve abnormal proliferation of myeloid cells in bone marrow. Examples include polycythemia vera, chronic myelogenous leukemia (CML), and myelofibrosis (rarely). In addition to these, other cancers like chronic lymphocytic leukemia (CLL), certain lymphomas that affect mature lymphocytes are also characterized by leukocytosis. In rare cases, certain endocrine diseases or conditions lead to abnormal increase in the number of eosinophils, basophils and lymphocytes. Endocrine factors lead to an increased release and migration of certain white blood cells from bone marrow to tissues, thereby giving rise to increase in the number of circulating WBCs. Examples and the cell types increased are: » Basophil: Hypothyroidism, ovulation, and estrogen abnormalities » Eosinophil: Addison's disease » Neutrophil: Polycystic Ovary Syndrome (PCOS) » Lymphocytes: Thyrotoxicosis and Addison's disease Some of the additional factors that have been associated with a high white blood cell count include: » Medicines » Smoking » Stress » Cardiac problems The precise set of clinical presentations as well as symptoms associated with a high WBC count vary with the etiological factor, cell type increased, age, gender and medical history of the individual. These factors determine the treatment options which vary from antibiotics, anti-inflammatory medications to chemotherapy and radiation therapy. In extreme cases, exchange blood transfusion may be required. A routine investigation of blood counts is essential to for an early diagnosis, especially in case of chronic leukemias since they manifest mild symptoms which are often ignored. Disclaimer: This Buzzle article is for informative purposes only, and should not be used as a replacement for professional medical advice.