Dapple, yes the birth control pill is, in fact, an immunosuppressant steroid. People who are on steroids in general have a lower immune system, and the pill isn't too much different. If you do some research on the internet and specifically look for scientific articles about steroid sex hormones, you will see that the pill is an immunosuppressant.
That's part of what made me decide to quit. When I read some of the stuff that I saw, I was very concerned because I didn't realize the business about being a steroid. So I quit and I'm a lot healthier all around, thankfully!