Tetanus is often derived from horse serum, which is a heterologous protein that many people might be allergic to. This is why a sensitivity test is done before administering tetanus shots; a positive allergy test indicates the presence of this antigen in your body, which can lead to an allergic reaction. To prevent such allergic reactions, desensitization is performed through intramuscular injections, which means administering the tetanus vaccine in three separate doses. This approach helps avoid allergies or even severe anaphylactic shock that might occur from the tetanus shot.