general_pathology:immunology:adaptive_immunity

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What provides the second signal to initially activate naive B-cells responding to a soluble antigen to start producing IgM?

Complement C3b binding to CD21 on the B cell

B7 proteins binding to CD28 on the B-cell

Complement C3b binding to CD79b on the B cell

Complement C3b binding to CD79a on the B cell