Causes of an elevated white cell count, or leukocytosis, include diseases of the bone marrow or the immune system, reactions to drugs and leukemia, says Mayo Clinic.The patient may also be fighting off a viral or bacterial infection or suffering an allergic reaction. Other things that raise white blood cell counts are polycythemia vera, myelofibrosis, smoking, tuberculosis, and physical and emotional stress.
People who have had their spleens removed also experience an increase in their white blood cell count for a period of time, according to the American Family Physician. Whooping cough can also cause leukocytosis, says Mayo Clinic.
Though it is a slow growing blood cancer that causes the overproduction of red blood cells, polycythemia vera also causes the overproduction of white blood cells, says Mayo Clinic. This leads to complications such as bleeds and blood clots. Myelofibrosis is another rare blood cancer where the bone marrow starts to overproduce abnormal blood cells, including white blood cells. This not only crowds out healthy cells of all types but causes scarring in the bone marrow itself. This further lowers the production of healthy blood cells.
A white blood cell count in an adult that's over 11,000 per microliter of blood is considered to be high, according to the American Family Physician. A count of over 100,000 per microliter is considered a medical emergency because of the high risk of hemorrhage and stroke.