**Core Concept**
Graft-versus-host disease (GVHD) is a major complication of allogenic hematopoietic stem cell transplantation, where the immune cells from the donor (graft) recognize the recipient's body as foreign and attack it. This is mediated by T lymphocytes that are present in the donor's bone marrow.

**Why the Correct Answer is Right**
GVHD is primarily caused by T cells, specifically CD4+ and CD8+ T cells, which recognize the recipient's major histocompatibility complex (MHC) molecules as foreign. The recipient's MHC molecules are recognized as non-self by the donor's T cells, triggering an immune response. The T cells then proliferate and release cytokines, leading to inflammation and tissue damage in various organs.

**Why Each Wrong Option is Incorrect**
**Option A:** Macrophages play a role in the immune response, but they are not the primary cells responsible for GVHD. Macrophages are involved in the phagocytosis of foreign particles and the presentation of antigens to T cells.
**Option B:** B cells are not primarily involved in GVHD. While B cells can participate in the immune response, their role is more focused on antibody production rather than cell-mediated immunity.
**Option C:** Natural killer (NK) cells can contribute to GVHD, but they are not the primary cells responsible. NK cells are involved in the innate immune response and can recognize and kill target cells that lack MHC class I molecules.

**Clinical Pearl / High-Yield Fact**
GVHD can be acute or chronic, with acute GVHD typically occurring within the first 100 days after transplantation and chronic GVHD occurring later. The risk of GVHD can be reduced by using immunosuppressive medications and matching the donor and recipient for MHC molecules.

**Correct Answer: C. Natural killer cells.