Leucopenia is a decrease in the number of white blood cells, which puts a person at risk for infection.  Normally when a person has infection or inflammation, the number of white blood cells increases so there are more cells to fight the infection with.  In some cases though, the infection actually destroys white blood cells, leaving the number decreased.  White blood cells can be destroyed due to other causes such as some medications, automimmune disorders and disorders of the bone marrow.  If no cause for the low white blood cell count can be determined, your physician may refer you to a hematologist.  The hematologist will need to do further tests to determine the cause of the leucopenia.  If you have a low white blood cell count, you should practice good handwashing and watch for signs of infection.