The information for prednisone can be found at
http://www.nlm.nih.gov/medlineplus/druginfo/medmaster/a601102.htmlThe only herb it specifically mentions is St John's Wort but as with all drugs it does advise that you tell your doctor any herbal preparations you are taking. It also does state that calcium and potassium supplements should be taken if using prednisone. If you read through the side effects of prednisone you will see why reducing the dose to coincide with transfusions only, rather than daily, is so important. Prednisone is a drug to avoid taking regularly (daily) long term. The "take only with transfusion" approach has proven to be more effective and far less harmful.
As Danielle mentioned, caution should be taken with aloe as the combination with prednisone can lower potassium levels. It may be possible to take it if you are also supplementing with potassium but consult with your doctor first. On the other hand, wheatgrass is a food. It is a sprouted grain of wheat and the only side effect I have ever heard of was in someone who got a bit too carried away with the fresh juice and was taking too much and it gave some intestinal distress. Nothing harmful, but definitely uncomfortable. Wheatgrass is a bioflavanoid, which are also found in many fruits and vegetables and are considered beneficial for health. It is also a good anti-oxidant so it may have some benefit in protecting cells against damage done by iron and the many drugs thal patients take. It is reputed to have a positive effect on the health of the immune system and it seems that if anything, it may lessen the antibody reaction by helping to restore proper immune system function. I would not hesitate to use wheatgrass.