**Core Concept**
CD56 is a neural cell adhesion molecule (NCAM) that serves as a marker for natural killer (NK) cells and some T cell subsets. It plays a role in cell adhesion and signaling, particularly in the context of immune cell interactions.

**Why the Correct Answer is Right**
CD56 is specifically expressed on the surface of NK cells, which are key components of the innate immune system. These cells recognize and eliminate virally infected cells or tumor cells without prior antigen exposure. The expression of CD56 on NK cells facilitates their ability to adhere to and interact with other immune cells, such as dendritic cells and T cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because CD56 is not a marker for B cells. B cells express different surface molecules, such as CD19 and CD20.
**Option B:** This option is incorrect because CD56 is not a marker for macrophages. Macrophages express different surface molecules, such as CD68 and CD163.
**Option C:** This option is incorrect because CD56 is not a marker for endothelial cells. Endothelial cells express different surface molecules, such as CD31 and CD144.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that CD56 is not only a marker for NK cells but also for some T cell subsets, such as TcR gamma/delta T cells. This knowledge is crucial for understanding the complex interactions between different immune cells in various disease states.

**Correct Answer: C. Natural Killer Cells**