The new drug that seems to have obliterated Jimmy Carter’s tumors is part of a revolutionary approach to cancer therapy. It’s called immunotherapy and the idea behind it is to marshal the body’s own defenses and aid them in the fight against cancer. Dr. Natalie Azar helps explain what that means.

Q: What is immunotherapy?

A: Immunotherapy helps the body’s own immune system to battle harder and smarter against cancer cells. One reason the body has a tough time mounting an attack on cancer is that tumors are composed of our own cells. Those cells have mutated in a way that allows them to grow out of control. But because they are our own cells, the immune system may not see them as foreign invaders that need to be destroyed. One approach has been to find ways to show the immune system that the cancer cells are invaders and must be targeted and destroyed.

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Former U.S. President Jimmy Carter takes questions from the media during a news conference about his recent cancer diagnosis and treatment plans, at the Carter Center in Atlanta, Georgia, in this file photo taken August 20, 2015. Carter told a Sunday School class at his church in Georgia that his cancer was gone, the Atlanta Journal-Constitution reported on Sunday, citing a church member. REUTERS/John Amis/Files JOHN AMIS / Reuters

Q: How did Jimmy Carter’s therapy work?

A: One way that cancers evade the body’s defenses is by hijacking parts of the immune system. In this case, certain proteins keep it from attacking normal cells along with the damaged ones. Researchers have learned that tumors can commandeer these proteins, which work like a set of brakes to dampen the body’s immune response. A drug that blocks these “brakes” may boost the immune system’s ability to destroy cancer cells. The drug Carter was given is approved by the Food and Drug Administration and is called Keytruda. Two other drugs have been approved by the FDA that work in similar ways.

Q: Who is a candidate for these types of drugs?

A: These drugs are currently being used to treat people with advanced melanoma and some types of advanced lung cancer. But there is some evidence that this type of drug could work with bladder, colon, kidney and some types of head and neck cancers.

Q: How successful is this kind of treatment?

A: Studies have shown that 30 to 40 percent of patients with advanced melanoma will have a good response to these drugs and 30 percent will be long-term survivors. Doctors have had even better success — between 50 and 60 percent — when combining different immunotherapy treatments. But there is a downside to taking multiple drugs: more side effects.

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Q: How much do these drugs cost?

A: They’re very expensive, though perhaps not as costly as you might think. According to the Melanoma Research Foundation, for an infusion center to buy Keytruda it costs $12,000 per month, or $150,000 per year per patient. The good news is that insurance will cover the drug as long as you are taking them for an approved use.