i've mentioned the other possible mechanism of action, which definately seems more plausible - to me anyway.
Unfortunately, the "low level pathogen" suggestion doesn't fly either. The OP was complaining of hayfever. As far as I remember that's caused by a certain type of antibody (IgE) binding the pollen and causing some cells of your immune system to activate and release inflammatory mediators and histamines (that's what hayfever treatments contain anti-histamines). If you just introduce more of the pollen then you'll get more of a reaction. If you give them something that is different but gives the same symptoms it'll....give the same symptoms. Assuming it hasn't been diluted out.