**Question:** NK cells CD marker is

A. CD56
B. CD16
C. CD3
D. CD8

**Core Concept:** Natural Killer (NK) cells are a type of white blood cell that plays a crucial role in the immune system, particularly in fighting viral infections and tumor cells. NK cells express certain cell surface markers, which help identify and characterize them.

**Why the Correct Answer is Right:** CD56, also known as NKG2D, is the correct CD marker for NK cells. CD56 is a cell surface marker that is expressed by NK cells and other immune cells, such as T cells and B cells. It is involved in the activation and regulation of immune responses, particularly in the context of NK cell function.

**Why Each Wrong Option is Incorrect:**

A. CD3: CD3 is a complex of cell surface proteins found on T cells, not NK cells. NK cells do not express CD3, making option C incorrect.

B. CD16: CD16 is a Fc receptor found on NK cells, but it is not the primary CD marker for NK cells. While CD16 is important for NK cell function, CD56 is more specific for identifying NK cells.

C. CD3: As mentioned earlier, CD3 is a marker for T cells, not NK cells. This makes option C incorrect.

D. CD8: CD8 is a co-receptor found on cytotoxic T cells, not NK cells. NK cells express different markers for identification, making option D incorrect.

**Clinical Pearl:** Understanding the correct CD markers for immune cells is essential for medical professionals, as it helps in identifying and characterizing specific cell types during diagnostic or research purposes. This knowledge is crucial for understanding immune system function, immune responses, and disease pathogenesis.