NK cell marker is?
**Core Concept**
The question is testing the student's knowledge of natural killer (NK) cells, a type of lymphocyte that plays a key role in the innate immune response. 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 express a unique set of surface markers, including CD56 and CD16. CD56, also known as neural cell adhesion molecule (NCAM), is a glycoprotein that is present on the surface of NK cells and some other immune cells. CD16, on the other hand, is a low-affinity receptor for the Fc portion of IgG antibodies. The combination of CD56 and CD16 expression makes NK cells highly effective at recognizing and destroying cells that have been opsonized by antibodies.
**Why Each Wrong Option is Incorrect**
**Option A:** CD4 - This is incorrect because CD4 is a marker for T helper cells, not NK cells.
**Option B:** CD8 - This is incorrect because CD8 is a marker for cytotoxic T cells, not NK cells.
**Option C:** CD19 - This is incorrect because CD19 is a marker for B cells, not NK cells.
**Clinical Pearl / High-Yield Fact**
NK cells play a critical role in the defense against viral infections, such as influenza and herpes simplex virus. They can also contribute to the development of autoimmune diseases, such as lupus, when they are overactive or dysregulated.
**Correct Answer: C. CD56. CD56 is a marker for NK cells.**