(thÃ»r'mÄ«'Én) An electrically charged particle or ion that is emitted by a heated conducting material. The electrons emitted from the cathodes of electron tubes (such as cathode ray tubes) are thermions.
A low white blood cell count (leukopenia) is a decrease in disease-fighting cells (leukocytes) in your blood. Leukopenia is almost always related to a decrease in a certain type of white blood cell (neutrophil). The definition of low white blood cell count varies from one medical practice to another. In general, for adults a count lower than 4,000 white blood cells per microliter of blood is considered a low white blood cell count. For children, that threshold varies with age.
A low number of WBCs is called leukopenia. A WBC count below 4500 is below normal. One type of white blood cell is the neutrophil. This type of white blood cell is important for fighting infections. An adult with fewer than 1700 neutrophils in a microliter of blood or 1.7 x 10^9/L has a low white blood cell count.