Most of the studies relating to cocoa allergy are from factory workers and chocolatiers who spend all of their time working with the product.
There is a 2S seed storage protein in cocoa which is similar to those proteins found in tree nuts. Chocolate contains theobromine and caffeine which may cause people to believe they are allergic to cocoa.
I have put together some more comprehensive resources at http://www.allergyresources.co.uk/Cocoa.php
The allergy resources page for cocoa covers the key allergens, which allergic syndromes are linked to this allergy and other foods which are commonly cross reactive with cocoa .
There is also regularly updated links to useful websites specifically for cocoa 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.