False regarding NK cells:
**Core Concept**
Natural Killer (NK) cells are a type of lymphocyte that plays a crucial role in the innate immune response, particularly in the defense against viral infections and tumor cells. NK cells are characterized by their ability to recognize and kill infected cells or tumor cells without prior antigen exposure.
**Why the Correct Answer is Right**
NK cells recognize and kill target cells through a mechanism involving the activation of the perforin-granzyme pathway, which leads to cell lysis. This process is mediated by the expression of activating receptors on the surface of NK cells, such as NKG2D and NKp46, which interact with ligands on the surface of target cells. The activation of NK cells also involves the inhibition of inhibitory receptors, such as CD94/NKG2A, which helps to regulate the balance between activation and inhibition.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because NK cells can also recognize and kill target cells through antibody-dependent cellular cytotoxicity (ADCC), which involves the engagement of Fc receptors on the surface of NK cells by antibody-bound antigens.
**Option B:** This option is incorrect because NK cells are not primarily involved in the humoral immune response, which is mediated by B cells and antibodies.
**Option C:** This option is incorrect because NK cells can also recognize and kill target cells through the recognition of stress-induced ligands, such as MICA and MICB, which are upregulated on the surface of stressed or infected cells.
**Clinical Pearl / High-Yield Fact**
NK cells are an important component of the innate immune response and play a crucial role in the defense against viral infections and tumor cells. The activation of NK cells can be enhanced by cytokines, such as IL-2 and IL-12, which are often used in immunotherapeutic strategies to boost the immune response against cancer.
**Correct Answer:** C.