B-lymphocyte antigen CD19 is a surface protein of the immunoglobulin superfamily. It binds to the B cell receptor on the cell surface of B cells and lowers the threshold of its activability. Thus, overexpression of CD19 in mice, as well as knockout mice, shows that CD19 affects the signaling threshold of receptors that regulate B lymphocyte selection, activation, and differentiation. At the molecular level, CD19 is involved in the activation of phosphatidylinositol 4,5-bisphosphate 3-kinases (PI3K). It is glycosylated and phosphorylated. Translated with www.DeepL.com/Translator (free version)