Final answer:
Allergic contact dermatitis caused by exposure to toxic plants such as poison ivy, oak, or sumac is a type IV hypersensitivity reaction.
Step-by-step explanation:
Allergic contact dermatitis caused by exposure to toxic plants such as poison ivy, oak, or sumac is a type IV hypersensitivity reaction. This reaction is also known as delayed hypersensitivity and involves a cell-mediated immune response.
Upon initial exposure, the immune system becomes sensitized and memory helper T cells are produced. On subsequent exposures, memory helper T cells are reactivated, releasing inflammatory cytokines that stimulate macrophages and cytotoxic T cells. This inflammatory response leads to the development of a rash or dermatitis.
Unlike immediate hypersensitivity reactions, which involve antibody-mediated immune responses (type I, II, and III hypersensitivity), delayed hypersensitivity reactions (type IV hypersensitivity) are characterized by the involvement of T cells and take about one to two days to manifest.