**Core Concept**
Oval cells are a type of liver cell, also known as hepatocytes, that are involved in the regeneration of liver tissue. They are characterized by their oval shape and are found in the liver's parenchyma.

**Why the Correct Answer is Right**
Oval cells are a type of liver progenitor cell that has the ability to differentiate into both hepatocytes and cholangiocytes. They are activated in response to liver injury, such as that caused by hepatitis or cirrhosis, and play a key role in the regeneration of liver tissue. Oval cells express specific markers, such as cytokeratin 19 and epithelial cell adhesion molecule (EpCAM), which distinguish them from mature hepatocytes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because oval cells are not typically associated with pancreatic tissue. The pancreas has its own distinct cell types, including beta cells, alpha cells, and acinar cells.
**Option B:** This option is incorrect because oval cells are not a type of immune cell. While the liver does contain immune cells, such as Kupffer cells and hepatic macrophages, oval cells are a type of liver epithelial cell.

**Clinical Pearl / High-Yield Fact**
Oval cells are an important factor in the liver's ability to regenerate itself in response to injury. Understanding the role of oval cells can help clinicians to develop effective treatments for liver diseases, such as liver cirrhosis and liver cancer.

**Correct Answer: D. Oval cells are seen in liver regeneration.**