## **Core Concept**
The question tests knowledge of B cell markers, which are crucial for identifying and classifying B lymphocytes. B cells express specific surface molecules (markers) that can be used to distinguish them from other immune cells. These markers are important in both research and clinical settings for diagnosing and monitoring diseases affecting the immune system.

## **Why the Correct Answer is Right**
CD19, CD20, and CD21 are well-known markers expressed on the surface of B cells. **CD19** is a marker for B cells from the early stages of development until maturity, making it a reliable B cell marker. **CD20** is another specific marker for B cells, present from the early pre-B cell stage until maturity, but it is lost upon differentiation into plasma cells. **CD21**, also known as CR2 (complement receptor 2), is expressed on mature B cells and follicular dendritic cells, playing a role in the activation of B cells. On the other hand, **CD8** is primarily a marker for a subset of T lymphocytes known as cytotoxic T cells, not B cells.

## **Why Each Wrong Option is Incorrect**
- **Option A:** CD19 is indeed a B cell marker.
- **Option B:** CD20 is a B cell marker.
- **Option C:** CD21 is a B cell marker.

## **Why Option D is Correct (and thus the statement about it being an exception is right)**
- **Option D:** CD8 is not a B cell marker; it is a marker for cytotoxic T cells.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the loss of specific B cell markers can be used to monitor the progression of certain diseases, such as B cell malignancies. For instance, in chronic lymphocytic leukemia (CLL), B cells often express CD19, CD20, and CD5. Understanding these markers helps in the diagnosis and management of the disease.

## **Correct Answer:** D. CD8.