Final answer:
T cells are primarily responsible for type IV (delayed-type) hypersensitivity reactions.
Step-by-step explanation:
The primary cell responsible for type IV (delayed-type) hypersensitivity reactions is T cells. Specifically, it is the sensitized T cells of the Th1 class that play a key role in this type of immune response. Upon re-exposure to an antigen, these sensitized T cells release cytokines that recruit macrophages and other phagocytes to the site, leading to a cellular immune response that takes 24 to 72 hours to develop.