How does Lee use Calpurnia's actions in dealing with Tim Johnson to show the stuble discrimination prevalent in small Southern towns?

Calpurnia is the first to recognize the serious nature of the situation, and she immediately makes the right phone calls, and runs to warn the neighbors. She protects many from danger, but receives no praise in comparison to Atticus who actually kills the dog. Though Atticus's skill with a gun is remarkable, Calpurnia's swift action and knowledge are invaluable. This is a reflection of how the black community's assistance to the white community in Maycomb is often unacknowledged.