Have you received an answer on this yet? There are a number of things that could cause a low white blood cell count. A low white blood cell count usually is caused by one of the following:
Viral infections that disrupt bone marrow function
Congenital disorders characterized by diminished bone marrow function
Cancer or other diseases that damage bone marrow
Autoimmune disorders that destroy white blood cells or bone marrow cells
Overwhelming infections that use up white blood cells faster than they can be produced
Drugs that destroy white blood cells or damage bone marrow
This website lists the specific causes. I hope it helps you!
http://www.mayoclinic.com/health/low-white
-blood-cell-count/MY00162/DSECTION=causes