Allergic Contact Dermatitis is classified as which type of hypersensitivity reaction?

Allergic Contact Dermatitis is classified as a Type 4 hypersensitivity reaction, also known as delayed-type hypersensitivity. This type of reaction is mediated by T-lymphocytes (a type of white blood cell) rather than antibodies. In the case of allergic contact dermatitis, the process begins when a sensitizing agent, such as certain metals, chemicals, or plants, comes into contact with the skin and modifies proteins, making them appear foreign to the immune system.

Upon initial exposure, T-cells become sensitized to these modified proteins. On subsequent exposures, these memory T-cells proliferate and mount an immune response, leading to inflammation and the characteristic symptoms of contact dermatitis, which typically present 24 to 48 hours after re-exposure. This delayed response distinguishes Type 4 hypersensitivity from the other types, which involve different mechanisms.

Type 1 hypersensitivity reactions (e.g., anaphylaxis) involve IgE antibodies and are immediate, while Type 2 reactions involve antibody-mediated cell destruction and Type 3 reactions involve immune complex formation. The unique characteristics of Type 4 hypersensitivity, specifically the role of T-cells and delayed reaction time, confirm why allergic contact dermatitis is classified as such.