The lineage specific marker for B cells is
**Core Concept**
B cells, a type of lymphocytes, play a crucial role in the adaptive immune system by producing antibodies. Lineage-specific markers are essential for identifying and characterizing these cells. CD19 is a transmembrane glycoprotein specifically expressed on the surface of B cells from the early stages of B cell development until maturity.
**Why the Correct Answer is Right**
CD19 is a crucial marker for B cells due to its early and exclusive expression on these cells. It is involved in the activation of B cells by binding to the CD21 molecule (also known as CR2) on follicular dendritic cells. This interaction facilitates the activation of B cells and the subsequent differentiation into antibody-secreting plasma cells. CD19 also plays a role in B cell receptor signaling by interacting with the B cell receptor complex.
**Why Each Wrong Option is Incorrect**
**Option B:** CD20 is expressed on mature B cells and is often used as a marker for B cell malignancies. However, it is not as lineage-specific as CD19, as it is not expressed on early B cells.
**Option C:** CD21 (also known as CR2) is primarily expressed on follicular dendritic cells and is involved in the activation of B cells. However, it is not a lineage-specific marker for B cells.
**Option D:** CD22 is a sialic acid-binding Ig-like lectin (SIGLEC) expressed on mature B cells, but it is not as specific for B cells as CD19.
**Clinical Pearl / High-Yield Fact**
It is essential to remember that CD19 is expressed on B cells from the early stages of development, making it a more lineage-specific marker than CD20. This distinction is crucial for diagnosing B cell malignancies and understanding the normal development of the immune system.
✓ Correct Answer: A. CD 19