Although they can be minimised, allergic reactions cannot be totally excluded. However, through correct management their duration and effect can be largely controlled.
The action you need to take will vary depending upon the severity of the allergic reaction. In any case you should remain calm as stress can make the symptoms worse. If you are suffering a mild allergic reaction such as sneezing, watery eyes or itchy skin etc. it is likely that using your usual antihistamine or prescribed medication will help and this may be all you need to do. Remember it will take time to become effective so it is advisable to take your medication at the early onset and remove any allergens if possible.
If however the symptoms are affecting breathing with swelling in the throat or tongue then this could be a severe allergic reaction and urgent medical help will be required.