Cell-mediated immunity (CMI): In which of the following immunologic scenarios does CMI play a central and necessary role?

Introduction / Context:
Cell-mediated immunity (CMI) is driven primarily by T lymphocytes and activated macrophages rather than by circulating antibodies. Recognizing clinical settings where CMI dominates helps clinicians anticipate pathology and select appropriate diagnostic tests or therapies.

Given Data / Assumptions:

• Delayed-type hypersensitivity (DTH) is a T cell–mediated response.
• Allograft rejection requires T cell recognition of foreign MHC/peptides.
• Graft-versus-host reaction arises from donor T cells attacking host tissues.

Concept / Approach:
CMI involves CD4+ Th1 cells, CD8+ cytotoxic T cells, and activated macrophages. DTH (e.g., tuberculin skin test) is a hallmark Th1-mediated reaction. Transplant immunology centers on T cell recognition leading to cytotoxicity and cytokine-mediated damage. GVH is likewise T cell driven, but initiated by donor cells in an immunocompromised host.

Step-by-Step Solution:
Map each listed condition to T cell–mediated mechanisms. Confirm antibody is not the primary effector in these processes. Integrate the three scenarios under the umbrella of CMI. Choose the inclusive option “All of the above.”

Verification / Alternative check:
Histology of rejected grafts and GVH lesions shows T cell infiltration and cytokine signatures. DTH testing relies on T cell memory rather than serum antibody levels.

Why Other Options Are Wrong:

• Any single choice alone is incomplete.
• Immediate hypersensitivity only: Type I hypersensitivity is IgE-mediated and not representative of CMI.

Common Pitfalls:
Confusing Type II/III antibody-mediated reactions with CMI; transplant and DTH reactions are classic T cell phenomena.

Final Answer:
All of the above.