In humans as well as in dogs, the condition is genetic.
Normally dogs will develop itchy skin and scratch, lick and bite to try and get relief. If not treated, the dogs can scratch so hard that they create sores that can become infected, which usually require antibiotics to clear up.
If the signs of atopy are seasonal and last fewer than three months, oral medications (like cortisone) may be used to control itching. In more severe cases, dogs are given a skin allergy test to pinpoint the specific allergens your dog is sensitive to.
They can then receive injections of the allergic material in minimal doses, which will allow the dog to build up immunities in his system. There are other treatment options such as immunotherapy, antihistamines, steroids, and medicated shampoos.
If you think that your dog is suffering from seasonal allergies, make an appointment for him with his veterinarian.
Source: Adapted from the American Animal Hospital Association