## **Core Concept**
T cells play a crucial role in graft rejection by recognizing antigens presented by the major histocompatibility complex (MHC) on the surface of donor cells. This recognition is fundamental to the cell-mediated immune response. The specific antigens recognized by T cells in the context of graft rejection are primarily related to the MHC molecules.

## **Why the Correct Answer is Right**
T cells recognize antigens presented by MHC molecules on antigen-presenting cells (APCs). In the context of graft rejection, the MHC molecules on the donor cells are recognized as foreign by the recipient's T cells. This recognition leads to an immune response against the graft. The correct answer, **MHC (Major Histocompatibility Complex)**, is right because T cells specifically recognize and respond to the MHC molecules on the surface of donor cells, which are polymorphic and vary between individuals.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While some T cell responses can be modulated by interactions with costimulatory molecules, the primary recognition of antigens in graft rejection is not through these molecules but through the MHC.
- **Option B:** Non-MHC antigens can also trigger an immune response, but T cells primarily recognize MHC molecules in the context of graft rejection.
- **Option C:** This option seems to be a placeholder and does not provide a recognizable antigen or molecule related to T cell recognition in graft rejection.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **MHC compatibility** is crucial in organ transplantation to minimize the risk of graft rejection. T cells are central to this process, and their recognition of MHC molecules as foreign is a primary trigger for the immune response against the graft. Matching donor and recipient MHC types as closely as possible can significantly reduce the risk of rejection.

## **Correct Answer:** B. MHC.