The American Botanical Council is very excited about a proprietary Panax quinquefolius preparation (Cold-FX) and has released a comprehensive monograph on this herbal product. The bottom line: Cold-FX seems to be quite good at preventing colds and flus, when taken at moderate doses, and even effective in managing symptoms once the cold has started, when taken at much higher doses. The best research that isn't partially funded by the manufacturers comes out of Canada and is a trial involving over 300 people - and the results are indeed interesting.
I reviewed the pharmacology of this product, and it seems to isolate the polysaccharide fraction from ginseng roots. There has been a good amount of research around these compounds (Clinical Microbiology Review, e.g.), and some have proposed that complex sugars such as rhamnose, galactose and arabinose present in genera such as Larix, Echinacea, Andrographis, Eleutherococcus, and Panax are at least partially responsible for their immune-enhancing power. Human studies seem to point at increased proliferation of CD4 T-cells and NK cells of the immune system, although why this happens still remains a mystery. Perhaps the polysaccharides have some similarities to the sugar-like antigens on the surface of bacterial cell walls - but that is simply speculation.
In sum, I think this is further evidence that dietary polysaccharides have a positive effect on human immunity. But don't rely on expensive proprietary products to get these chemicals into your diet: consider an excellent soup stock made of bones and/or seaweeds, astragalus, reishi, and ginseng (American and/or Siberian). You can freeze this preparation in ice cube trays after simmering it for 4-6 hours in a crock pot, and take 2-3 ice cubes in warm water during the cold and flu season. This is a tried-and-true, ancient (minus the ice cubes) way of using these excellent herbs.