- Nickel-containing dental alloys are commonly associated with Type IV hypersensitivity reactions, which are also known as delayed-type hypersensitivity reactions.
- This type of immune response is mediated by T lymphocytes rather than antibodies.
- When nickel ions come into contact with the skin or mucosal tissues, they act as haptens, binding to proteins and forming complexes that are recognized as foreign by sensitized T cells.
- This leads to the activation of these T cells and subsequent release of inflammatory cytokines, causing symptoms such as contact dermatitis or mucosal inflammation, typically developing 24-72 hours after exposure.

In contrast, the other types of hypersensitivity reactions involve different immune mechanisms:
- Type I hypersensitivity is an immediate allergic reaction mediated by IgE antibodies and mast cell degranulation (e.g., anaphylaxis, urticaria).
- Type II hypersensitivity involves antibody-mediated cytotoxicity directed against cells or tissues (e.g., autoimmune hemolytic anemia).
- Type III hypersensitivity results from immune complex deposition leading to inflammation (e.g., serum sickness).

Because nickel-induced reactions are primarily cellular and delayed, Type IV hypersensitivity is the most common mechanism implicated in dental alloy-related allergic reactions.

Reference: J Oral Pathol Med, Volume 45, Chapter on Oral Hypersensitivity Reactions / Page 32-45