Many vets see allergy cases by the thousands. Routine steroids, anti-inflammatories, antibiotics, salves, creams and ointments are prescribed, although their success is variable and the price the animal's body pays may be very steep.
Allergies happen when the body's immune system goes overboard and overreacts to something.
This can be a multitude of things from grass to pollen to food to chemicals to laundry detergent. Your animal may be allergic to something and show it right away, or it may take time to build up an allergy to something. Also, something else entirely may trigger an allergic reaction to something else, for example, your animal companion may be allergic to beef and not show visible signs, however when fed corn with that beef, the allergic reaction will arise. Once the immune system sees something they recognize as foreign or bad, the gears are set into motion. Sometimes they are very obvious - after running through the fields your dog develops a red itchy rash.