**Core Concept**
Langerhans cells are a type of dendritic cell present in the skin and mucosa, playing a crucial role in the initiation of immune responses. They are part of the immune system's surveillance mechanism, recognizing and processing foreign antigens.

**Why the Correct Answer is Right**
The histologic hallmark of Langerhans cells is the presence of Birbeck granules, also known as Langerhans cell granules. These unique organelles are rod-shaped or tennis racket-shaped structures that are found within the cytoplasm of these cells. Birbeck granules are involved in the processing and presentation of antigens to T-cells, facilitating the initiation of immune responses. The presence of Birbeck granules is a diagnostic feature of Langerhans cells and is used in histopathological examination to identify these cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specify the characteristic feature of Langerhans cells.
**Option B:** This option is incorrect as it refers to a different type of immune cell, not Langerhans cells.
**Option C:** This option is incorrect as it is a characteristic of a different type of cell, not Langerhans cells.

**Clinical Pearl / High-Yield Fact**
Langerhans cells are named after Paul Langerhans, a German pathologist who first described these cells in the late 19th century. They play a crucial role in the initiation of immune responses and are involved in the pathogenesis of various skin diseases, including lichen planus and sarcoidosis.

**Correct Answer: C. Birbeck granules.**