**Core Concept**
The Graft-Versus-Host Disease (GVHD) is a major complication of allogenic hematopoietic stem cell transplantation, where the immune cells of the donor attack the tissues of the recipient. GVHD involves a complex interplay of immune cells, cytokines, and various cell types.

**Why the Correct Answer is Right**
The Graft-Versus-Host reaction involves the activation of donor T cells, which recognize the recipient's tissues as foreign. This leads to the production of cytokines such as TNF-α, IFN-γ, and IL-2, which mediate the inflammatory response and tissue damage. The recipient's cells, including epithelial cells, fibroblasts, and endothelial cells, are targeted by the donor T cells, leading to tissue injury and dysfunction.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because the recipient's immune system is typically suppressed in the context of GVHD, making it less likely to be involved in the reaction.

**Option B:**
This option is incorrect because while the recipient's cells are affected in GVHD, the primary effector cells are the donor T cells.

**Option C:**
This option is incorrect because the graft itself is not the primary target of the GVHD reaction, but rather the recipient's tissues.

**Clinical Pearl / High-Yield Fact**
It's essential to note that GVHD can be acute or chronic, with the latter being more common and often involving the skin, liver, and gastrointestinal tract.

**Correct Answer:** C.