It's important to understand the diseases that are being protected against, the kind of vaccine being used, and the possible side effects so that you can discuss your dog's vaccination schedule with your veterinarian and make an informed decision. Think of the body's immune system as an army, whose job it is to repel foreign invaders. When a virus or bacteria enters the body, the immune system recognizes the enemy and mobilizes an army of antibodies to kill the intruder. Once the war is over, the immune system remembers that particular invader, and will respond quickly if that invader shows up again. Giving an animal a vaccine, made of either dead organisms or organisms that have been modified so they can't cause the disease, gives the immune army a preview of the enemy so it will be ready to leap into action and destroy the bad guys if the dog is exposed to them.
Allergic reactions occur when the army overreacts at the time of re-exposure.
This overreaction can cause facial swelling, hives, vomiting, diarrhea and rarely shock and death. Repeated exposure may cause mild reactions to become dangerous ones. Autoimmune disease is rather like an army having a mutiny, the immune system begins to destroy its own body. There is some thought that repeated exposure to vaccines may be a trigger for autoimmune diseases. After weaning, these antibodies gradually wear out and the puppy must make his own antibodies.