Auto-immune disorders such as Lupus can often cause low WBC's. It sounds as thought the Imuran (azathioprine) is not necessarily helping you with this part of your disease.
Other similar medications that may be of help are Cellcept and MTX. These, along with Imuran, are the 3 medications most often used (after Plaquenil and Prednisone) to control Lupus. I wouldn't be surprised if your Dr doesn't try one of these soon - or maybe you can ask about
them. It isn't known who or why one person is helped by one of these 3 and another is not -- we have to keep trying until the medication is properly matched to the patient.
Lynnwood, Lupus & Sjogren's Moderator, Dx: 2002DIAGNOSING LUPUS & HW's LUPUS 101
"Life is far too important to be taken seriously." - Oscar Wilde