**Core Concept**
Kupffer cells are specialized macrophages that play a crucial role in the reticuloendothelial system, primarily functioning as phagocytic cells in the liver. They are involved in the removal of foreign substances, bacteria, dead cells, and other debris from the bloodstream.

**Why the Correct Answer is Right**
Kupffer cells line the sinusoidal spaces within the liver lobules, where they can effectively capture and destroy pathogens and other foreign particles. This process is essential for maintaining the liver's detoxification and metabolic functions. The presence of Kupffer cells in the liver explains why it is often referred to as a "filter" for the blood.

**Why Each Wrong Option is Incorrect**
**Option A:** While macrophages are indeed found in various organs, Kupffer cells are specifically located in the liver.
**Option B:** Microglial cells are the brain's resident macrophages, not Kupffer cells.
**Option C:** Hepatocytes are the main cell type in the liver, responsible for metabolic functions, not phagocytosis.
**Option D:** Langerhans cells are a type of dendritic cell found in the skin, not Kupffer cells.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Kupffer cells are a key component of the reticuloendothelial system, which also includes the spleen and lymph nodes. This system plays a vital role in immune surveillance and the removal of pathogens from the body.

**Correct Answer:** C. Liver