Final answer:
The rash from poison ivy is caused by a type IV hypersensitivity reaction, also known as delayed hypersensitivity or contact dermatitis. It involves memory helper T cells that become reactivated upon secondary exposure, leading to inflammation and the typical rash.
Step-by-step explanation:
The itchy and blistering rash caused by contact with poison ivy is the result of a type IV hypersensitivity reaction. This reaction involves a cell-mediated immune response which is a delayed hypersensitivity, typically manifesting one to two days after secondary exposure to the allergen. On first contact with poison ivy, there's no visible reaction, but the body becomes sensitized, creating memory helper T cells. Upon subsequent exposure, these cells are reactivated, triggering the release of inflammatory cytokines that stimulate other immune cells like macrophages and cytotoxic T cells, resulting in the characteristic rash of poison ivy, technically known as contact dermatitis.