## **Core Concept**
The recognition of antigens by T cells is a crucial step in the immune response, including graft rejection. T cells recognize antigens presented by major histocompatibility complex (MHC) molecules on the surface of antigen-presenting cells (APCs). In the context of graft rejection, the antigens recognized by T cells are primarily those expressed on the surface of the graft.

## **Why the Correct Answer is Right**
T cells recognize **MHC molecules** (also known as human leukocyte antigens or HLA in humans) on the surface of antigen-presenting cells (APCs) in the context of graft rejection. MHC molecules are responsible for presenting peptide fragments from inside the cell to T cells. There are two main types of MHC molecules involved in T cell recognition: MHC class I, which presents peptides from proteins produced inside the cell and is recognized by CD8+ T cells, and MHC class II, which presents peptides from proteins produced outside the cell and is recognized by CD4+ T cells. The disparity in MHC molecules between the donor graft and the recipient's immune system is a primary trigger for graft rejection.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is blank and does not provide any information to evaluate.
- **Option B:** This option is also blank and cannot be assessed.
- **Option C:** Similarly, this option does not contain any information to consider.
- **Option D:** This option as well is blank.

## **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. The closer the MHC molecules match between the donor and recipient, the lower the risk of rejection. However, finding an exact match can be challenging, and immunosuppressive drugs are often used to prevent rejection.

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