Local reactions are those that occur at the injection site and consist of lumps, bumps and swellings where the vaccine was injected. They are rarely large or painful enough to cause the animal to limp or go off feed. Usually they are mild and go away without treatment. Systemic reactions are those that affect the whole animal. Fortunately, these types of reactions are also rare.
Allergic reactions are the most serious.
They occur in an animal whose immune system overreacts to the vaccine and produces a very severe reaction. They can happen almost instantly after the product is injected or they can be delayed for up to several hours. The treatment for allergic reactions is an injection of antihistamine, steroids or epinephrine. Many producers keep a bottle of one or more of these drugs on hand when vaccinating, along with instructions from their vet on the proper use and dosage. Ideally, an animal suspected of having an allergic reaction would be examined and treated by a vet, but sometimes there isn't time and the producer must treat the animal him or herself.