What are possible allergic reactions to braces?

Very rare. Nickel allergy and latex allergy. Latex is the number one, our gloves and elastics are made from latex. We use an alternative when allergy is present. Brackets and wires are made of metal alloys that contain some percentage of nickel. I check for this by asking what type of jewelry the patient can and cannot wear. Invisalign is an alternative, also titanium braces are available.
Few. Nickel is the principal allergen involved in the fabrication of many braces. In addition, nickel is part of nickel-titanium wires used in conjunction with braces. Some self-ligating braces contain only titanium, which has proven to create negligible allergic response. Latex elastics used in conjunction with braces can cause allergic responses, but non-allergic vinyl options are available.
Few. Braces that contain nickel may cause oral dermatitis, but rarely. If suspected prior, I have patient place a brace on their abdomen with a band-aid to hold it close; if after a few days no irritation, then no allergy. A more common allergy would be to latex which is used to tie the wire to the brace. I avoid this by using braces that tie together with a titanium gate: called self-ligating braces.
Nickel allergy. Stainless stel contains nickel and skin contact can cause irritiation and rash. A common clinical finding is a small irritating rash below the navel caused by nickel in the blue-jean button or snap. Dermatologists typically perform patch testing if confirmation of this sensitivity is required.