One supplement I used to buy was called ImmunoLin. It's basically blood plasma from cows that's had the Immunoglobullin G extracted (igG) along with a lot of other immune boosting stuff. I think Patrick Arnold discovered it and NOW Foods used to sell it, but I really couldn't find many vendors selling it now. But the stuff worked great. Here's a link to the company that owns the patent:
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All the other suggestions are good too, along with a good multi, plenty of Vit C, washing your hands thoroughly all the time (OCD style). Also, if you're getting upper respiratory infections, I would recommend checking out a nasal spray product called Xlear. It's saline solution combined with xylitol, which has been shown in studies to kill the bacteria in the nasal passages. You can find it at local health food shops and on Amazon. The company also sells gum, mouthwash, and xylitol powder which you can consume as an alternative sweetener (it's a sugar alcohol, so watch out for the GI problems if you use too much). But the studies show around 4g a day (if I remember correctly) can help kill bacteria.
A good probiotic supplement can help too. I use
Invalid Link Removed. They use all patented probiotics made by DuPont and other reputable companies and the product itself is made in Japan. They have lots of studies on the strains of probiotics they use showing that they reduce upper respiratory infections, boost the immune system, reduce allergies, etc. The capsules are manufactured so the probiotics don't degrade or the bacteria don't die during shipment and storage like a lot of other probiotics do. Beyond that, yogurt is another good alternative.
There was something else I was going to mention, but it slipped my mind. Anyway, hope that helps,
StarScream
It increases white blood cells hence the stronger immune system. One of the best aas ever imo. Clean lean gains with no sides... And you feel great on it.
Unfortunately, a lot of people report real serious hair loss from Primo, and it's such a rare compound it's hard to find.