Poppy seeds have been found to contain profilin allergens, those especially sensitive to these proteins may have allergic reactions to poppy seeds.
Allergic reactions caused by eating poppy seeds are sometimes thought 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.
Recently a study has found that poppy seeds contain 2S albumin proteins, more commonly found in nuts and seeds. Individuals sensitive to these proteins may react to the 2S albumin proteins in the seeds.
I have put together some more comprehensive resources at http://www.allergyresources.co.uk/Poppy.php
The allergy resources page for poppy seed covers the key allergens, which allergic syndromes are linked to this allergy and other foods which are commonly cross reactive with poppy seed.
There is also regularly updated links to useful websites specifically for poppy seed 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.