Where did mad cow disease come from, and how does it avoid antibodies?

Mad cow disease: From wikipedia: researchers are not completely sure how cows get mad cow disease, but they believe it comes from certain food that was given to cows. Some of this food contains the remains of dead cows that had the infection causing the cows that are eating it to get the infection. Mad cow disease affects the cows brain causing them to go "mad." the brain lacks antibody protection.