Aniseed is a spice, also known as anis. As a very rare allergy it is not well studied and there has been no confirmation of which allergens are causing the reactions.
Allergic reactions 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.
I have put together some more comprehensive resources at http://www.allergyresources.co.uk/Aniseed.php
The allergy resources page for aniseed covers the key allergens, which allergic syndromes are linked to this allergy and other foods which are commonly cross reactive with aniseed .
There is also regularly updated links to useful websites specifically for aniseed 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.