## **Core Concept**
Natural killer (NK) cells are a type of lymphocyte that play a critical role in the innate immune system. They are known for their ability to recognize and destroy tumor cells and virus-infected cells without prior sensitization. NK cells act by inducing cell death through mechanisms like apoptosis.

## **Why the Correct Answer is Right**
NK cells primarily target cells that have lost or have altered expression of Major Histocompatibility Complex (MHC) class I molecules, which are typically expressed on the surface of healthy cells. Tumor cells or virus-infected cells may downregulate MHC class I expression to evade recognition by cytotoxic T cells. However, this downregulation makes them more susceptible to NK cell-mediated cytotoxicity because NK cells can recognize the lack of MHC class I expression as a signal to attack. This mechanism is known as "missing self" recognition.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because NK cells do not primarily target cells based on the presence of MHC class II molecules. MHC class II molecules are primarily involved in antigen presentation to T helper cells.
- **Option B:** This option is incorrect because while NK cells can interact with various cell types, their primary mechanism does not involve targeting cells based on the presence of MHC class I molecules alone; rather, it's about the absence or alteration of MHC class I expression.
- **Option D:** This option is incorrect because the endothelium, while it can interact with immune cells, is not the primary target for NK cell cytotoxicity in the context provided.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that NK cells are crucial in the defense against viral infections and in surveillance against tumor cells. Their function can be enhanced by certain cytokines, such as interleukin-2 (IL-2), which is used in immunotherapy regimens for certain cancers.

## **Correct Answer:** C. Tumor cells.