Which type of allergic reaction occurs slowly and is not dose-related?

Type IV allergic reactions, also known as delayed-type hypersensitivity, are characterized by their slow onset, which typically occurs hours to days after exposure to an allergen. This type of reaction does not depend on the amount of allergen; instead, it involves T-cell mediated responses, where T lymphocytes (a type of white blood cell) recognize antigens presented by antigen-presenting cells. Unlike other types of hypersensitivity reactions, which may produce symptoms immediately or are directly related to the dose of the allergen, Type IV reactions build up over time and can result in chronic inflammation.

In this context, Type I reactions are immediate and mediated by IgE antibodies, which are related to the dose of the allergen. Type II reactions involve IgG or IgM antibodies and typically occur through mechanisms like cytotoxicity that can be dose-dependent. Type III reactions also involve immune complexes and can manifest quickly, distinguishing them from the slower Type IV reactions. Thus, the defining characteristics of delayed onset and a lack of direct dose relationship firmly categorize Type IV as the correct answer.