**Core Concept**
Kupffer cells are specialized macrophages located in the liver, playing a crucial role in the reticuloendothelial system. They are responsible for phagocytosing foreign substances, bacteria, dead cells, and other debris from the bloodstream. This process is essential for maintaining liver function and overall immune defense.

**Why the Correct Answer is Right**
Kupffer cells are found in the liver sinusoids, where they line the walls and engulf foreign particles and waste products. Their presence in the liver allows for the efficient removal of toxins and pathogens from the bloodstream, protecting the body from potential harm. The liver's rich blood supply and extensive vascular network enable Kupffer cells to access and interact with a large volume of blood.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because Kupffer cells are not primarily found in the spleen, although splenic macrophages share similar functions.
**Option B:** This is incorrect because lymph nodes contain lymphocytes and macrophages, but not specifically Kupffer cells.
**Option C:** This is incorrect because the lungs contain alveolar macrophages, but these are distinct from Kupffer cells.

**Clinical Pearl / High-Yield Fact**
Kupffer cells are an essential component of the liver's immune defense, and dysfunction can contribute to various liver diseases, including cirrhosis and liver cancer.

**Correct Answer: C. Liver. Kupffer cells are specialized macrophages found in the liver sinusoids, playing a crucial role in the reticuloendothelial system.**