Which of the following cell surface markers is required for lysis of IgG-coated target cells (antibody-dependent, cell-mediated cytotoxicity, or ADCC) by natural killer cells?
**Core Concept**
The process being tested is antibody-dependent, cell-mediated cytotoxicity (ADCC), a mechanism employed by natural killer cells to lyse antibody-coated target cells. This process involves the binding of antibodies to target cells and subsequent recognition and engagement of these antibodies by immune effector cells like natural killer cells.
**Why the Correct Answer is Right**
Natural killer cells utilize the Fc receptor, specifically FcγRIIIA (CD16), to recognize and bind to the Fc region of IgG antibodies. This interaction allows natural killer cells to engage and lyse target cells that have been opsonized by IgG antibodies, thereby facilitating ADCC. The binding of IgG antibodies to FcγRIIIA triggers a series of intracellular signaling events that ultimately lead to the activation of natural killer cells and the release of cytotoxic granules.
**Why Each Wrong Option is Incorrect**
* **Option A:** FcεRI is primarily involved in the recognition of IgE antibodies and the activation of mast cells and basophils in allergic reactions. It is not involved in ADCC by natural killer cells.
* **Option B:** CD4 is a co-receptor on T helper cells that plays a crucial role in the recognition of MHC class II molecules. It is not involved in ADCC by natural killer cells.
* **Option D:** CD28 is a co-stimulatory receptor on T cells that provides a second signal for T cell activation. It is not involved in ADCC by natural killer cells.
**Clinical Pearl / High-Yield Fact**
Natural killer cells play a critical role in the innate immune response, particularly in the defense against viral infections and tumor cells. The FcγRIIIA receptor is a key component of this process, and polymorphisms in the gene encoding this receptor have been associated with susceptibility to certain infections and autoimmune diseases.
**Correct Answer:** C. CD16. CD16 is the correct answer, as it is the Fc receptor responsible for the recognition and binding of IgG antibodies by natural killer cells in ADCC.