According to The Beekeeper's Handbook by Diana Sammataro and Alphonse Avitabile, "The compounds in propolis that have some pharmacological (antiviral, antibacterial, antioxidant, antifungal, anti-inflammatory, and antihistamine) activities are quercetin, pinocembrin, caffeic acid, phenethyl ester, acacetin, and pinostrobin. . . . None of its curative properties have been extensively studied in the United States."
From what I've read in this and other sources, propolis certainly has antimicrobial properties, which is why you can sometimes find propolis at health food stores and such, but as far as information on actual usage for helping the human immune system, I believe there is minimal to no information on the subject, as far as scientific studies goes.