**Core Concept**
The **Major Histocompatibility Complex (MHC)** plays a crucial role in the immune system, and its expression is vital for the proper functioning of the immune response. **MHC class I** and **class II** molecules are key components of this complex. The **MMR (Mismatch Repair)** system is involved in maintaining genomic stability, but in this context, we are looking at the expression of **MMR proteins** in relation to the immune system, particularly in the context of **cancer** and **immune surveillance**.

**Why the Correct Answer is Right**
The correct answer is related to the expression of MMR proteins in certain cell types. MMR proteins are typically expressed in cells that are involved in the immune response, such as **lymphocytes** and **antigen-presenting cells**. These proteins are crucial for the proper functioning of the immune system, particularly in the context of recognizing and responding to **tumor cells**.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while MMR proteins are involved in maintaining genomic stability, their expression is not limited to this cell type.
**Option B:** This option is also incorrect as MMR proteins are not exclusively expressed in this cell type.
**Option D:** Similarly, this option is incorrect as it does not accurately represent the primary site of MMR protein expression.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **deficiencies in MMR proteins** can lead to increased susceptibility to certain types of **cancer**, such as **Lynch syndrome**. This highlights the importance of MMR proteins in maintaining genomic stability and preventing tumor formation.

**Correct Answer:** Correct Answer: C. lymphocytes.