Broccoli is in the cabbage family of brassicas. No allergens have been detected in broccoli, but it has been shown to contain Lipid Transfer Proteins (LTP).
Broccoli also contains high levels of salicylates. These are naturally derived compounds in food and are made by the plant in defence against disease and attack by insects.
You may be suffering from LTP Syndrome if you have reactions to various fruits, vegetables and nuts and your reactions continue to be severe after you have discarded the peel and have cooked the food.
I have put together some more comprehensive resources at http://www.allergyresources.co.uk/Broccoli.php
The allergy resources page for broccoli covers the key allergens, which allergic syndromes are linked to this allergy and other foods which are commonly cross reactive with broccoli.
There is also regularly updated links to useful websites specifically for broccoli 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.