**Core Concept**
Natural killer (NK) cells are a type of lymphocyte that plays a crucial role in the innate immune response, particularly in the elimination of virally infected cells and tumor cells. They are characterized by their ability to recognize and kill target cells without prior sensitization.

**Why the Correct Answer is Right**
NK cells participate in antibody-dependent cellular cytotoxicity (ADCC) reactions, a mechanism that involves the binding of antibodies to antigens on the surface of target cells. The Fc region of these antibodies binds to Fc receptors on the surface of NK cells, which triggers a series of intracellular signaling events leading to the release of cytotoxic granules and the killing of the target cell. This process is an essential component of the immune response, particularly in the elimination of tumor cells and virally infected cells. The Fc receptors on NK cells are responsible for recognizing the Fc region of antibodies, which activates the cell for ADCC.

**Why Each Wrong Option is Incorrect**
**Option A:** NK cells are not a subset of T cells; they are a distinct lineage of lymphocytes that originate from the hematopoietic stem cell. While both T cells and NK cells are involved in immune responses, they have distinct functions and characteristics.
**Option B:** NK cells are not involved in type III hypersensitivity reactions, which are mediated by IgG antibodies and complement. NK cells are primarily involved in innate immunity, whereas type III hypersensitivity reactions are a component of adaptive immunity.
**Option C:** NK cells do not require prior sensitization to kill virally infected cells or tumor cells. In fact, one of the defining characteristics of NK cells is their ability to recognize and kill target cells without prior exposure or sensitization.

**Clinical Pearl / High-Yield Fact**
NK cells are an essential component of the innate immune response, and their dysfunction has been implicated in various diseases, including cancer and viral infections. Understanding the mechanisms of NK cell activation and function is crucial for the development of immunotherapies and cancer treatments.

**✓ Correct Answer: D. They participate in antibody-dependent cellular cytotoxicity reactions**