Allergic reactions to parsnip 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.
Parsnips also contain furocoumarins which can cause urticaria and contact dermatitis.
Another suggestion is that plants in the Apiaceae family contain profilin proteins which can cause allergic reactions in uncommon circumstances.
Allergy to parsnip is loosely linked to Pollen Food Allergy Syndrome. You may have Pollen-Food Allergy Syndrome if you suffer from parsnip allergy with oral allergy symptoms to 3 or more of the foods mentioned in the cross reactivity section.
If sensitised to Birch pollen you may have Pollen Food Allergy Syndrome and may also react to Apple, Carrot, Kiwi, Pear, Peach, Plum, Nectarine, Apricots, Cherries, Tomato, Celery, Potato, Parsley, Pepper, Cumin, Peas, Dill, Fennel, Hazelnut, Walnut, Almonds, Coriander, Peanuts, Lentils and Beans.
Parsnip is in the family Apiaceae, other foods in this family are coriander, caraway seed, celery, chervil, cumin, dill, fennel, parsley and aniseed.