Hello, Welcome to HCM, We never know to which drug we are allergic, but incidence of allergy to tetanus injection is very less. As your mother had few minor symptoms of allergy following tetanus injection, you reassure your mother nothing will happen because it is one of the safest drug. If the symptoms are still persisting after 24 hours following the tetanus shot ,i would like to advise 1. Tipid sponging 2.Apply cold pack over the hives 3.Oral antihistaminics This will improve the condition otherwise you have to take your mother to nearby doctor. Thank you.