The primary allergic reaction to tetanus is anaphylactic shock. Some patients may experience skin redness, itching, chest tightness, difficulty breathing, pale complexion, and cold sweats within minutes after receiving the tetanus vaccine. Some may also experience nausea, vomiting, and even abdominal pain. Additionally, blood pressure may drop, and the pulse may slow down. In severe cases, coma can occur, and even cessation of heartbeat and breathing. The second type of allergic reaction is serum sickness, which can occur after a tetanus injection, characterized by erythema, edema, and itching at the injection site.