I was just worked up for a low white cell count, specifically the neutrophils. My situation is a little different in that I'm not taking a drug known to lower the WBC like 6-MP but it is still a low WBC, with the same risks, i.e. the inability to fight infection.
The hematologist I saw said that my WBC, which just dropped from 30 years at 3-3.6 down to 2.2 was not too concerning. I had asked him about treatment with methotrexate which had been considered at one time and he said it would require dosing considerations and extra monitoring but it was not ruled out, even with a WBC this low starting the treatment. So, even though a normal WBC ranges from 4-11,000 it can go considerably lower before there are serious concerns. Even at a level of 2.2 I don't have a problem with infections