The FDA has been trying to regulate Supplements for a long time, but not for all the right reasons.
Also, there are fly by night vitamins just like anything else. The motto is "consumer beware". Vitamins produce results or people would not be using them.
Bill did point me toward web-MD of which I checked it out. I searched immune system, and immune system enhancement and found guide lines and info on both. Anyone serious about their health should look it up.
My take on this whole thing is, take care of yourself in the first place, eat right, exercise, and lay off the health robbing things in our eating habits that contribute to bad health, (too much sugar and too many simple carbs, white bread and the like.
I know it is easier said then done and things happen, high BP, high cholesterol. When this gets out of control from neglect, then go to the Doctor and take his pills, but, we also need to take control of our own fate and clean up our act.
There are people that are diagnosed with what ever, are over weight, get their meds, get their problem under control and never address the over weight problem which anybody will tell you, is one root cause of many issues. Another consideration is our food because of mass production simply is not the same vitamin rich product it was in earlier times. Trying to feed an ever growing population is daunting, to say the least. Hence, people in their quest for health turn to Vitamins and Supplements as a magic bullet. Health is a life style not a fad, take control yourself, there is a great place to start first.