LAURAN NEERGAARD, AP Medical Writer

WASHINGTON (AP) — A provocative study in mice suggests something as simple as breathing in extra oxygen might give immune cells a boost in fighting cancer.

Tumors put up defenses to block immune system attacks. One way: Low oxygen levels inside the tumor can spur production of a molecule that blocks tumor fighters called T cells. Researchers at Northeastern University wondered if adding oxygen to tumors could strip away that defense. So they put mice with lung tumors in chambers that let them breathe oxygen doses similar to what hospitals deliver through masks.

Tumors shrank more in the high-oxygen group, especially when researchers also injected the mice with doses of extra T cells.

They caution the approach must be tested in people to see if it really makes a difference.