**Core Concept**
Graft-versus-host disease (GVHD) is a complication that arises after allogeneic hematopoietic cell transplantation or solid organ transplantation. It occurs when the immune cells (T cells) in the transplanted graft (donor) recognize the recipient's (host) tissues as foreign, leading to an immune response against the host's cells.

**Why the Correct Answer is Right**
The correct answer is due to the presence of immunocompetent T cells in the graft tissue. These T cells are capable of recognizing and responding to the host's antigens, leading to the activation of an immune response. This is because T cells are the primary effector cells of the immune system responsible for cell-mediated immunity. In GVHD, the donor T cells see the recipient's tissues as foreign and mount an immune response, leading to tissue damage and disease.

**Why Each Wrong Option is Incorrect**
**Option B:** Immunocompetent T cells in host - While the host's immune cells are involved in the response to GVHD, it is the donor T cells in the graft that initiate the disease process.

**Option C:** Immunocompetent B cells in graft tissue - B cells play a role in the humoral immune response and are not primarily responsible for the cell-mediated immunity seen in GVHD.

**Option D:** Immunocompetent B cells in host - Like option C, B cells in the host are not responsible for the initiation of GVHD.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that GVHD can be acute or chronic and can affect any organ system, but the gut and skin are the most common sites of involvement. The diagnosis of GVHD is often based on clinical presentation, laboratory findings, and histopathological examination of affected tissues.

**✓ Correct Answer: A. Immunocompetent T cells in graft tissue**