## **Core Concept**
Graft-versus-host disease (GVHD) is a major complication of allogeneic hematopoietic stem cell transplantation. It occurs when immune cells from the transplanted graft recognize the recipient's body as foreign and mount an immune response against it. The cells primarily responsible for GVHD are those capable of cell-mediated immunity.

## **Why the Correct Answer is Right**
The correct answer involves **T lymphocytes (T cells)**, which are central to cell-mediated immunity. T cells can directly kill target cells or produce chemical signals that activate the immune response. In the context of GVHD, donor T cells can recognize the recipient's tissues as foreign and initiate an immune response, leading to damage of the recipient's skin, liver, and gastrointestinal tract. The presence of T cells in the graft is a significant risk factor for developing GVHD.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While **B cells** are crucial for humoral immunity and can produce antibodies, they are not the primary effector cells in GVHD. B cells can contribute to GVHD through antibody production, but their role is less direct compared to T cells.
- **Option C:** **Natural Killer (NK) cells** are involved in the innate immune response and can cause cytotoxic effects, but they are not the primary cells responsible for GVHD. NK cells can contribute to GVHD, especially in the setting of T cell-depleted transplants, but their role is secondary to T cells.
- **Option D:** This option might seem plausible due to the involvement of various immune cells in GVHD, but **T cells** are specifically implicated as the key effectors.

## **Clinical Pearl / High-Yield Fact**
A crucial point to remember is that **T cell depletion** of the graft can significantly reduce the risk of GVHD but may also increase the risk of graft failure and relapse of the underlying malignancy. This highlights the delicate balance in managing immune responses post-transplantation.

## **Correct Answer:** B. T lymphocytes.