Back in Venice, Shylock escorts Antonio to prison, accompanied by a jailer and Solanio. Shylock tauntingly tells the jailer not to have any mercy on Antonio, who is a fool who "lent out money gratis." Antonio begs Shylock for mercy, but Shylock cuts him off: "thou call'dst me dog before thou hadst a cause, / but since I am a dog beware my fangs" (3.3.6–7).
Shylock here admits that he is acting like an animal. But he insists that he is doing so because he has been forced into it by the Christian's own harsh and unfair treatment of him. They force him to act like a dog, then complain when he bites.
Antonio gives up on asking for mercy. He knows that Shylock wants revenge on him because he has paid off the debts of so many people who owed Shylock.
By paying off the debts of others, Antonio stopped Shylock from collecting interest. He thinks Shylock's anger stems only from monetary loss.
Solanio assures Antonio that the Duke won't allow Shylock's demand to be carried out. Antonio disagrees: "The Duke cannot deny the course of law," (3.3.26) or else he will discredit the justice of the state of Venice. Such an action, in turn would offend the many diverse "strangers" (3.3.27) upon whom the commerce of the city depends.
Antonio recognizes that Shylock has the law on his side. Notice that while Venetian Christians look down on foreigners, their city's wealth also relies on the trade of those "strangers," so the law must take precedence over the Venetian's prejudices.
Antonio jokes that he has grown so thin in his stress and grief that it will hardly be possible to cut a pound of flesh from him. Then, he urges the jailer on. If Bassanio comes to see him pay his debt, Antonio says, he does not care whether he dies or not.
Antonio's resignation to his fate, and even his thinness, makes him a Christ figure. Christ also went willingly, gently, to his death on the cross.