## **Core Concept**
The question tests understanding of Major Histocompatibility Complex (MHC) Class II molecule expression on antigen-presenting cells (APCs). MHC Class II molecules are crucial for the immune system, particularly in presenting peptide fragments from extracellular pathogens to T-helper cells. Typically, MHC Class II molecules are expressed on professional antigen-presenting cells.

## **Why the Correct Answer is Right**
Mature red blood cells, unlike other nucleated cells or professional APCs like dendritic cells, macrophages, and B cells, do not express MHC Class II molecules. This is because mature red blood cells lack a nucleus and most organelles, including those necessary for protein synthesis and antigen presentation. Their primary function is oxygen transport, not immune function.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Dendritic cells are professional antigen-presenting cells known to express MHC Class II molecules, which enable them to activate naive T cells.
- **Option B:** Macrophages are another type of professional APC that express MHC Class II molecules, facilitating their role in phagocytosis and antigen presentation.
- **Option C:** B cells, especially when mature and functioning as APCs, express MHC Class II molecules. This expression is vital for their interaction with T-helper cells.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while professional APCs like dendritic cells, macrophages, and B cells express MHC Class II, not all cells can present antigens to T cells. The absence of MHC Class II on certain cells, like mature red blood cells, limits their role in immune responses.

## **Correct Answer:** D. Mature Red Blood Cells