Allergic reactions can occur in dogs and can vary from mild to severe. The cause is often difficult to determine but can result from vaccinations or insect bites.
The most severe reaction is "anaphylaxis", an immediate reaction that leads to life-threatening symptoms that include vomiting, diarrhea, staggering, rapid drop in blood pressure, swelling of the larynx leading to airway obstruction, and cardiovascular collapse.
The most common type of allergic reaction is "Urticaria" which is a rapid swelling and redness of the lips, around the eyes, and in the neck region that can be very itchy. Hives may also occur. Urticaria may progress to anaphylaxis.
If treated successfully treated, the prognosis for long-term health is good. If your dog experiences signs of an allergic reaction, call your veterinarian immediately. They may recommend that you bring your dog right away. For mild reactions, they may recommend that you give your dog diphenhydramine (Benadryl®). The dose is approximately 1 mg per pound. Therefore, a 25 pound dog would get 25 mg or a 75 pound dog would receive 75 mg. Don't give any medications without the approval or direct recommendation of a veterinarian.
For more information, please read Alergic Reactions in Dogs.