Final answer:
Poison ivy primarily induces a Type IV (delayed-type) hypersensitivity immune response, activating CD4+ T cells and causing an inflammatory response.
Step-by-step explanation:
The immune response that poison ivy primarily induces, leading to the activation of CD4+ T cells and an inflammatory response, is Type IV (delayed-type) hypersensitivity.
During the initial exposure to poison ivy, sensitization occurs, stimulating helper T cells and resulting in the production of memory helper T cells. Upon secondary exposure, these memory T cells become reactivated and release inflammatory cytokines that stimulate macrophages and cytotoxic T cells, causing an inflammatory lesion at the exposed site.
This type of immune response is characterized by a cell-mediated immune response that takes approximately one to two days after secondary exposure to manifest fully.