Welcome to the forum. As far as pred lowering the immune system it does do that, but sometimes it's not enough to stop a flare. I think most of us here are on immuno suppressant drugs besides pred. I've had lupus for 21 yrs and I've taken every medication to treat lupus except for Arava and Gold.
Now, I'm really sick and have only worsened each year since 2001. In addition to having lupus I has RA and Fibro. I found out 2 yrs ago I have blood cancer in addition to everything. I'm so immuno suppressed and my body is still flaring I live with chronic pain 24/7. I don't believe I'll ever be well again. I need a bone marrow transplant to save my life. I think back and consider would I have taken the medication that I did that caused my cancer. I have to say I'm not so certain. Even though my Doctors did warn me about the risk I was taking.
Anyway, to answer your ? sometimes pred just isn't enough of a suppressant. If you need any more ?'s answered please ask away. You're gonna meet great ppl here. Again, welcome and looking forward to getting to know you.
dx fibro, SLE, glaucoma, cateracts, bells palsy, depression, migraine headaches, gastreopaersis, chronic anemia, RA,MDS (Blood Cancer). Tons of meds.