CD19 is a B cell-specific transmembrane glycoprotein belonging to the immunoglobulin superfamily, playing a crucial role in the development and function of B cells. It, along with CD21, CD81, and CD225, constitutes part of the B cell receptor complex, primarily expressed on the surface of B cells, with expression from early B cell to mature B cell stages. By enhancing B cell receptor signaling, CD19 promotes B cell activation, proliferation, and antibody production, involving key signaling pathways such as PI3K/Akt and MAPK. Its specific expression is essential for maintaining normal B cell function, while dysregulated expression or mutations are closely associated with B cell-related diseases such as immunodeficiency and leukemia, making CD19 an important target for treating these conditions.