Nothing in medicine is as straightforward as that. Most cancer treatments end up wiping out or suppressing many different kinds of cells, and a temporary suppression of white blood cells is considered an acceptable side effect. AIDS will have to be cured. A strong immune system often attacks donor organ cells, so the immune system is intentionally weakened. We may eventually develop solutions for all three issues, but I don't expect it to happen in our lifetimes. Heck, I predict we'll be cloning body parts in labs to obviate the need for anti-rejection meds long before we learn to fine-tune our immune response.