**Core Concept**
Natural killer (NK) cells are a type of lymphocyte that plays a crucial role in the innate immune system, particularly in the defense against viral infections and tumor cells. They are capable of recognizing and killing target cells without prior antigen exposure.

**Why the Correct Answer is Right**
NK cells recognize target cells through a combination of activating and inhibitory signals. Activating signals are triggered by the presence of stress-induced ligands such as MHC class I downregulation, whereas inhibitory signals are mediated by MHC class I self-ligands on healthy cells. When the activating signals outweigh the inhibitory signals, NK cells are activated to release cytotoxic granules and induce cell death.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might seem plausible, but NK cells do not primarily function through antibody-dependent cellular cytotoxicity (ADCC). While they can participate in ADCC, their primary mechanism of action is through direct recognition of target cells.
**Option B:** NK cells do not function as antigen-presenting cells (APCs). APCs are responsible for processing and presenting antigens to T cells, whereas NK cells primarily function through direct cell killing.
**Option C:** NK cells do not produce antibodies. Antibody production is a function of B cells, whereas NK cells are involved in cell-mediated immunity.

**Clinical Pearl / High-Yield Fact**
NK cells can be activated in response to tumor cell lysis, leading to a phenomenon known as "fratricide" or "activation-induced cell death" (AICD). This process can result in the elimination of autoreactive lymphocytes and help maintain immune tolerance.

**Correct Answer: D.**