Adverse reactions stemming from the use of fixed and removable orthodontic appliances have been a cause for concern for both orthodontists and researchers in the health care field. Nickel often has been indicated as a biological sensitizer capable of causing short- and long-term reactions (Type IV immune response). This article addresses the clinical and diagnostic aspects of nickel-related reactions in orthodontic patients, based on the literature. A case report highlighting the periodontal, clinical, and histopathologic aspects of an allergic patient in orthodontic treatment is presented.