In hamlet Whom does the ghost exempt from his call for revenge and why?

Hamlet's father, the Ghost, exempts Gertrude, Hamlet's mother, from his call for revenge. The Ghost states, "Leave her to heaven / And to those thorns that in her bosom lodge, / To prick and sting her."

The Ghost does not call for revenge against Gertrude because he believes she is already suffering from the guilt of her actions. He says that she is already being punished by the "thorns" of her conscience.