What characterizes type 4 hypersensitivity reactions?

Type 4 hypersensitivity reactions are primarily characterized by T-cell-mediated responses. In this type of hypersensitivity, the immune response is not driven by antibodies but rather by T cells, particularly CD4+ T helper cells and CD8+ cytotoxic T cells. When an individual is exposed to an allergen or antigen, these T cells become activated, leading to a delayed immune response.

This process can take several hours to days to manifest after exposure to the antigen, which distinguishes it from immediate hypersensitivity reactions, such as those involving IgE antibodies and mast cell degranulation. The activation of T cells results in the release of cytokines, which recruit and activate other immune cells, such as macrophages, contributing to tissue damage and inflammation at the site of antigen exposure.

The other aspects mentioned in the choices—immediate reactions, IgE involvement, and complement protein activation—are not characteristic of type 4 hypersensitivity, which emphasizes the role of T cells and the delayed nature of the response.