Final answer:
Type IV Hypersensitivity, or delayed-type hypersensitivity, is a delayed T-cell-mediated inflammatory reaction occurring 24 to 72 hours after antigen re-exposure, involving sensitized Th1 T cells and macrophage recruitment.
Step-by-step explanation:
The correct answer to the question of the mechanism of Type IV Hypersensitivity is a) Delayed-type hypersensitivity. This form of hypersensitivity is a delayed T-cell-mediated inflammatory reaction that takes a significant amount of time to manifest, usually between 24 to 72 hours, unlike the immediate reactions seen in Types I through III hypersensitivity. During the first encounter with an antigen, sensitization takes place. Upon re-exposure, the sensitized T cells, particularly of the Th1 subtype, facilitate a secondary cellular response by secreting cytokines. These cytokines recruit macrophages and other phagocytes to the site of the antigen, which may result in tissue damage or inflammation.
Delayed hypersensitivity is a cellular immune response and is the only type of hypersensitivity that is exclusively mediated by T cells, as opposed to Types I-III which involve B cell mediation.