## **Core Concept**
Non-Hodgkin lymphoma (NHL) is a type of cancer that originates from lymphocytes, which are a type of white blood cell. Follicular lymphoma is a common subtype of NHL characterized by the presence of neoplastic lymphocytes that typically express specific surface markers. Immunohistochemical staining is a crucial diagnostic tool for identifying these markers.

## **Why the Correct Answer is Right**
The correct answer, , is a marker for B cells. Follicular lymphoma arises from germinal center B cells. The neoplastic lymphoid cells in follicular lymphoma typically express B-cell-specific markers. **CD20** is a specific marker for B cells, and it is commonly used in the diagnosis and treatment of B-cell lymphomas, including follicular lymphoma. The neoplastic cells in follicular lymphoma usually retain the expression of CD20, making it a reliable marker for diagnosing this condition.

## **Why Each Wrong Option is Incorrect**
- **Option A:** . CD3 is a marker for T cells, not B cells. While T cells can be present in the lymph node, the neoplastic cells in follicular lymphoma are of B-cell origin.
- **Option B:** . CD15 is often associated with Hodgkin lymphoma and some types of non-Hodgkin lymphoma but is not specific for follicular lymphoma.
- **Option D:** . Bcl-2 is an anti-apoptotic protein that is overexpressed in follicular lymphoma due to the t(14;18) chromosomal translocation. However, the question specifically asks for a protein marker that neoplastic lymphoid cells would stain positively for, indicating a cell surface marker rather than an intracellular protein.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **CD20** is not only a diagnostic marker but also a therapeutic target. Rituximab, a monoclonal antibody against CD20, is a common treatment for follicular lymphoma and other CD20-positive B-cell lymphomas.

## **Correct Answer:** . **CD20**