D.W.
You can't build an immunity to an allergy - you can outgrow them, but you can't build an immunity like you can against common viruses, chicken pox, etc. via exposure.
If there were a valid natural cure/remedy for allergies, Zyrtec wouldn't be a billion dollar product, nor Claritin, etc. Having worked in the pharmaceutical industry for the past decade, I can assure you that nature is often the best source of medications, but most "natural remedies" that are passed via word-of-mouth have little to no scientific data behind them and are not FDA approved.
If it were my daughter, I'd have her take the Zyrtec for the 2 weeks. We actually started our 2 year-old on it on Monday because she wasn't responding to Claritin.
The pollen levels have been at record highs this season, but as we progress further into spring and summer, the pollen/mold levels should lessen along with the symptoms and reactions.