CD19 is a transmembrane glycoprotein that contains two extracellular immunoglobulin-like domains. CD19 is present in both benign and malignant B-cells and is considered to be the most reliable surface marker of this lineage over a wide range of maturational stages. In normal lymphoid tissue, CD19 is observed in germinal centers, in mantle zone cells, and in scattered cells of the inter-follicular areas.