Final answer:
The reaction to poison ivy that includes itching, blisters, and redness without scarring is a result of a Type IV hypersensitivity reaction, also known as delayed-type hypersensitivity.
Step-by-step explanation:
The type of hypersensitivity reaction that presents as urticaria and bullous eruptions with blisters that rupture and heal without scarring and is exemplified by the reaction to poison ivy is Type IV hypersensitivity. This type of hypersensitivity reaction is known as delayed-type hypersensitivity because the symptoms typically arise 48-72 hours after exposure to the antigen. The rash from poison ivy is caused by an immune response to the oil urushiol, which is a component of the plant. Upon the first exposure, there isn't a reaction, but the body becomes sensitized and generates memory T cells. On subsequent exposures, these memory T cells activate and trigger an inflammatory response, resulting in the classic symptoms of redness, itching, and blistering.