A less known effect of colloidal silver is in making the blood more slippery--platelets are less likely to clump together, helping prevent buildup (which can irritate the immune system) and damage blood flow. This is pretty cutting edge, I read about it here: http://www.sciencedaily.com/releases/2009/06/090601110403.htm
I also read that some allergens (pollens) may carry bacteria and viruses, and that can complicate the immune over-response. In Chinese medicine, silver is 'cooling' and is used to 'clear heat' which includes the inflamed conditions of a histamine attack.
It's interesting, too, that colloidal gold's primary medical action is on rheumatoid arthritis, which is also an auto-immune disease where the body attacks the joints. Is it attacking agglomerated particles in the joints? Gold may also help make things more slippery, it certainly seems to increase the conductivity in the nervous system, I'll go for silver and gold plated arteries, too!