What does Miss Pross do to protect Lucie and her child?What sacrifice must she make to defend them?

In A Tale of Two Cities, Miss Pross
is the staunch defender of Lucie Manette (later Lucie Darnay), her "ladybird," from the
first time we meet her to the last chapter of the novel.  In the beginning, Miss Pross
feels the need to defend her young charge from the "businessman" Jarvis Lorry.  Later,
once the family makes its way to Paris, Miss Pross has more serious threats with which
to contend. 

Once Lucie and Jarvis are on the road (with
Charles and young Lucie) heading out of Paris (and ultimately back to England), Madame
Defarge showed up at the apartment  ready to take Lucie into custody for an eventual
trial and beheading.  Jerry Cruncher has gone to get their hired carriage (for the two
of them, plus all the luggage), leaving Miss Pross alone with the angry Frenchwoman. 
She understands, despite the language barrier, that Madame Defarge is willing to kill
her to get to her ladybird.  The two women have a literal hand-to-hand fight, and the
gun which Madame Defarge produced goes off between them.  The result is a dead
Frenchwoman and a deaf Miss Pross.  She never regained her hearing from that
altercation, but she was glad to sacrifice for the family she
loved.