Allergic reactions to parsley are most likely to be caused by Bet v 1-like proteins, which cause reactions in people sensitised to birch tree pollen and give oral allergy type symptoms.
Lipid Transfer Proteins (LTP) have been identified in parsley, these proteins are resistant to heat and are found in many types of plants. Patients suffering from a more severe allergy to cooked fruit may be sensitised to this group of proteins.
I have put together some more comprehensive resources at http://www.allergyresources.co.uk/Parsley.php
The allergy resources page for parsley covers the key allergens, which allergic syndromes are linked to this allergy and other foods which are commonly cross reactive with parsley.
There is also regularly updated links to useful websites specifically for parsley 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.