What type of hypersensitivity is allergic contact dermatitis categorized as?

Allergic contact dermatitis is categorized as a Type IV hypersensitivity reaction, which is also known as delayed-type hypersensitivity. This form of hypersensitivity is mediated by T lymphocytes (specifically CD4+ T helper 1 cells) and does not involve antibodies. Instead, it occurs when a sensitizing substance (such as poison ivy, nickel, or certain chemicals) comes into contact with the skin and is processed by antigen-presenting cells.

After the initial exposure, sensitized T cells proliferate and, upon re-exposure to the same allergen, these activated T cells release cytokines that recruit and activate other immune cells, leading to an inflammatory response. This process typically takes 24 to 48 hours to develop after re-exposure, which characterizes the "delayed" aspect of the reaction.

In contrast, other types of hypersensitivity reactions, such as Type I, Type II, and Type III, involve different mechanisms, such as the immediate allergic response with IgE antibodies in Type I reactions, or antibody-mediated damage in Types II and III. Understanding these mechanisms helps clarify that allergic contact dermatitis is distinctively associated with the delayed immune response of Type IV hypersensitivity.