Shares of Bluebird Bio popped on Monday after the announcement of what CEO Nick Leschly called "very exciting data."

The biotech company and its partner, Celgene, released updated clinical results from an ongoing study of patients with relapsed/refractory multiple myeloma, a blood cancer.

In the trial active dose cohorts, 73 percent of evaluable patients received a very good partial response.

Leschly told CNBC's "Closing Bell" the people taking part in the study are "terribly sick."

"They've tried everything in the book and are sort of at the end of the line. And those patients have had an incredible response rate."

The idea is to take cells outside of the patient's body, use technology to "harness and direct it" and then put the cells back into the patient, he explained.

"We're really trying to harness the immune system to attack your cancer," Leschly said.

And while there are multiple drugs that have made a "big difference" for multiple myeloma patients, unfortunately the cancer comes back in a lot of patients, Leschly said.

"Then you have a very shortened lifespan on the order of 6 to 8 months. That's the need that we're talking about."

Bluebird Bio closed 8.5 percent higher at $91.30 on Monday.

—Reuters contributed to this report.