The marker for B lymphocyte is?
**Core Concept**
The question is asking about the specific marker or antigen present on the surface of B lymphocytes, which are a type of white blood cell involved in the adaptive immune response. The correct marker will help identify and differentiate B lymphocytes from other immune cells.
**Why the Correct Answer is Right**
CD19 is a transmembrane glycoprotein that serves as a specific marker for B lymphocytes. It is expressed on the surface of B cells from the early pro-B cell stage to the mature B cell stage, but not on other immune cells. CD19 plays a crucial role in the activation and differentiation of B cells by binding to CD21 (complement receptor 2) and CD81, facilitating the signaling events that lead to B cell activation and proliferation.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because CD20 is another marker found on B cells, but it is expressed on mature B cells, including plasma cells, whereas CD19 is present on immature and mature B cells.
* **Option B:** This option is incorrect because CD22 is a sialic acid-binding immunoglobulin-like lectin (Siglec) that is also expressed on B cells, but it is not as specific or sensitive as CD19 for identifying B cells.
* **Option C:** This option is incorrect because CD79a is a component of the B cell antigen receptor complex, but it is not a specific marker for B cells like CD19.
**Clinical Pearl / High-Yield Fact**
CD19 is a useful marker for identifying B cell malignancies, such as B cell acute lymphoblastic leukemia (B-ALL) and diffuse large B cell lymphoma (DLBCL), which often express high levels of CD19.
**Correct Answer:** C. CD19