The B Cell Co-Receptor, Composed of CD19/CD21/CD81, Is a Receptor $\alpha$

The B cell co-receptor, composed of CD19/CD21/CD81, is a receptor that binds to complement fragments such as C3dg. When an antigen bound by the BCR on a B cell has also been tagged with C3dg, the B cell co-receptor is stimulated together with the BCR. Signaling through the co-receptor:

A) Inhibits BCR signaling by leading to ITAM dephosphorylation
B) Inhibits BCR signaling by leading to PIP₃ dephosphorylation
C) Enhances BCR signaling by recruiting and activating PI 3-kinase
D) Enhances BCR signaling by bringing the Src-kinase together with Ig- $\alpha$ and Ig- $\beta$ .
E) Inhibits BCR signaling by sequestering the antigen away from the BCR.