All are true regarding NK cells except –
**Core Concept**
NK cells (Natural Killer cells) are a type of lymphocyte that plays a key role in the innate immune system, particularly in the recognition and destruction of virally infected cells and tumor cells. They can kill target cells without prior antigen exposure, making them a crucial component of the body's first line of defense.
**Why the Correct Answer is Right**
NK cells are characterized by their ability to recognize and bind to stressed or transformed cells through their activating receptors, such as NKG2D. This binding triggers the release of cytotoxic granules that contain perforin and granzymes, which ultimately lead to the apoptosis of the target cell. The Fc receptor on NK cells also allows them to recognize and destroy antibody-coated cells.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as NK cells do not primarily function through the complement system. While they may interact with components of the complement system, their primary mechanism of action is through the release of cytotoxic granules.
**Option B:** This option is incorrect as NK cells do not primarily function through the T-cell receptor. NK cells are a type of lymphocyte, but they do not express the T-cell receptor and instead use their own unique set of receptors to recognize and destroy target cells.
**Option C:** This option is incorrect as NK cells do not primarily function through the activation of macrophages. While NK cells may interact with macrophages and other immune cells, their primary function is to directly kill target cells.
**Clinical Pearl / High-Yield Fact**
NK cells are often impaired in patients with chronic viral infections, such as HIV, and in patients with certain types of cancer, such as leukemia. Understanding the role of NK cells in the immune response is crucial for the development of immunotherapies and cancer treatments.
**Correct Answer: B. NK cells do not primarily function through the T-cell receptor**