What type of hypersensitivity reaction is involved in allergic contact dermatitis?

Allergic contact dermatitis is characterized as a Type IV hypersensitivity reaction, which is a delayed-type hypersensitivity response predominantly mediated by T cells (specifically CD4+ T-helper 1 cells and CD8+ cytotoxic T cells) and does not involve antibodies.

This type of reaction typically occurs after sensitization to a specific allergen, which means that an individual first experiences exposure to a substance (like fragrances, metals, or other allergens) and, after a period, develops a memory T cell response. Upon re-exposure to the same allergen, the memory T cells become reactivated, resulting in an inflammatory response in the skin, which usually manifests as redness, swelling, and itching.

Type I hypersensitivity (like allergic rhinitis or anaphylaxis) involves IgE-mediated responses and immediate reactions, which do not explain the delayed manifestations seen in allergic contact dermatitis. Type II hypersensitivity is characterized by antibody-mediated destruction of cells, such as in hemolytic anemia, and is also not applicable in this scenario. Type III hypersensitivity involves the formation of immune complexes leading to inflammation and tissue damage, exemplified by conditions like systemic lupus erythematosus, again differing significantly from the mechanism of allergic contact dermatitis.

Thus,