TI - CD19 regulates Src family protein tyrosine kinase activation in B lymphocytes through processive amplification . AB - CD19 regulates constitutive and antigen receptor -induced signaling thresholds in B lymphocytes through its unique cytoplasmic domain . Herein , we demonstrate a novel molecular mechanism where interactions between CD19 and Lyn amplify basal and antigen receptor -induced Src family kinase activation . Lyn expression was required for CD19 tyrosine phosphorylation in primary B cells . Experiments with purified proteins demonstrated that CD19-Y513 was Lyn's initial phosphorylation and binding site . This led to processive phosphorylation of CD19-Y482 , which recruited a second Lyn molecule , allowing for transphosphorylation and amplification of Lyn activation . In vivo , CD19 deficiency impaired , and CD19 overexpression enhanced , Lyn kinase activity . Thus , CD19 functions as a specialized adapter protein for the amplification of Src family kinases that is crucial for intrinsic and antigen receptor -induced signal transduction .