Which type of hypersensitivity reaction is involved in contact dermatitis?

Contact dermatitis is primarily associated with a Type IV hypersensitivity reaction, which is also known as delayed-type hypersensitivity. This involves a T-cell mediated immune response rather than the immediate response seen in other types of hypersensitivity reactions.

In contact dermatitis, an individual is exposed to a sensitizing substance (the allergen) that leads to the activation of memory T cells upon subsequent exposures. The delayed nature of this reaction occurs because it takes time for T cells to proliferate and mount an immune response against the allergen. This process typically manifests in such conditions as allergic contact dermatitis, where erythema, swelling, and itching occur several hours to days after exposure—hence the term "delayed."

The hallmark of a Type IV hypersensitivity reaction is that it does not involve antibodies. Instead, it relies on the participation of lymphocytes and macrophages to mediate the inflammatory response. This distinguishes it from other types of hypersensitivity reactions, such as Type I, which is IgE-mediated and involves immediate release of histamine and other mediators, or Types II and III, which mostly involve antibody-mediated destruction of cells or immune complex formation, respectively.

Thus, the classification of contact dermatitis as a Type IV hypersensitivity reaction is fundamentally rooted in