T-lymphocytes drive cell-mediated graft rejection

Steps:

• Graft rejection occurs when the immune system detects foreign MHC antigens on donor tissue.
• T-lymphocytes recognize these antigens via direct cell-cell contact, unlike B-lymphocytes which rely on antibodies.
• Activated T-cells proliferate and migrate to the graft site to destroy it.
• This process is cell-mediated immunity, not humoral (antibody-based).

Why D is correct:

• T-lymphocytes circulate in blood and infiltrate the graft site to mount a cytotoxic response, as defined in cell-mediated immunity.

Why the others are wrong:

• A: Rejection is primarily T-cell mediated; B-cells and antibodies play a secondary role in some cases.
• B: T-lymphocytes stimulate B-cells but directly reject grafts via cytotoxicity, not antibody production.
• C: T-lymphocytes release cytokines, not antibodies; antibody production is a B-cell function.

Final answer: D