(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.
When someone's white blood cell count is low, his immune system isn't as strong as usual and he's at increased risk of infection, a condition known as neutropenia. The lower his white blood cell count is -- and the longer it stays low -- the higher the risk that he'll get sick or contract some type of infection. (Normal white blood cell counts range from 4,500 to 11,000 WBCs per cubic millimeter of blood. For more information, see understanding low white blood cell counts. Fever is often the first sign of infection, so keep an eye on his temperature. Call his doctor if his temperature climbs above 100 degrees.