## **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 through cell-to-cell contact and release of cytotoxic granules.

## **Why the Correct Answer is Right**
NK cells primarily target cells that have reduced or lost expression of major histocompatibility complex (MHC) class I molecules, which are typically present 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 killing, as NK cells can recognize the lack of MHC class I molecules as a signal to destroy the cell.

## **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 mainly expressed on antigen-presenting cells (APCs) like dendritic cells, macrophages, and B cells, and are involved in the activation of T helper cells.
- **Option B:** This option is incorrect as it does not accurately describe the primary target of NK cells. While NK cells can interact with various types of cells, their main function is not described by this option.
- **Option C:** This option is incorrect because while NK cells can interact with antibody-coated cells through antibody-dependent cellular cytotoxicity (ADCC), the question seems to focus on the direct recognition and killing by NK cells, not ADCC.

## **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 activity can be enhanced by certain cytokines, such as interleukin-2 (IL-2), which is a strategy used in some immunotherapies.

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