There are 7 allergens associated with allergy to birch pollen. The most important of which is called Bet v 1. These proteins are sometimes also called PR-10 as they are used by the plant in pathogenesis (defence against disease).
Birch pollen occurs in the UK between March and June, peaking in April. This varies in different countries. It is considered to have high allergenicity.
I have put together some more comprehensive resources at http://www.allergyresources.co.uk/Birch.php
The allergy resources page for birch pollen covers the key allergens, which allergic syndromes are linked to this allergy and other foods which are commonly cross reactive with birch pollen.
There is also regularly updated links to useful websites specifically for birch pollen allergies and the latest research papers on the topic.
If you have multiple allergies and are interested in how they are linked or want to find out more about food allergies outside the usual top 14 then you can follow me on Facebook or Twitter, where I share a food allergy card every day of the week.