CNBC Explains: Immuno-what?

The year's biggest event in cancer research, ASCO, or the American Society of Clinical Oncology meeting, kicks off Friday in Chicago, and is bound to send stocks flying early next week on data being presented.

Analysts say the key theme will be immunotherapies, drugs that harness the immune system to fight cancer. Companies including Bristol-Myers Squibb, Merck, AstraZeneca and Novartis, as well as smaller biotechs like Incyte, Juno Therapeutics and Kite Pharma, are all working in the space.

Read MoreThis year, at ASCO, all eyes are on pharma

For Bristol-Myers alone, BMO analyst Alex Arfaei estimates the drugs could bring in $12.9 billion in peak annual revenue.

ASCO will provide an updated glimpse of the promise of these medicines. So how do they work? Check out the video above.

Read MoreThese stocks are moving on cancer research news

—By CNBC's Meg Tirrell.

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