Pollen allergy

I searched on Google to see if I could find any pages about pollen allergy. And I found, as you might think, many web sites! Here are some of them:

This web page, http://www.hoptechno.com/book46.htm, was rather informative, although very similar to an article from Wikipedia. It gives you the bases on what allergy is.
At the end it discusses how to find treatments for allergy, e.g. immunotherapy. Many approaches to find a cure is through exposure, e.g. of nasal tissue. At this stage of my knowledge, are there other ways to find out what causes allergy? Or is it only by exposure with allergens?

Yet another website, medicinenet.com, talks about to what kind of pollen you could develop hay fever (also known as pollen allergy). Shortly it says that the heavier the pollen, the more likely to cause hay fever.

A typical American website is pollen.com.