CD19 is a central response regulator of B lymphocyte signaling thresholds governing autoimmunity.

The CD19/CD21 complex is categorized among the 'response regulator' class of receptors which determine the magnitude and outcomes of B cell receptor signals. Small changes in CD19 expression have dramatic effects on signaling thresholds within B cells, which in turn has considerable impact on the balance between humoral immune responses and tolerance induction. B cell signaling thresholds lowered by increased CD19 expression may significantly augment host susceptibility to the development of autoimmunity. Signals generated by C3d-antigen complex binding to CD21 may also be involved in the development of autoimmunity by regulating CD19 function. Since CD19 serves as a central regulator of signaling thresholds in B cells, the CD19/CD21 complex may be an appropriate target for suppressing the development of autoimmunity.