Molecules called reactive oxygen species, which include so-called “free radicals,” have long been viewed as damaging byproducts of our body’s metabolism—a reason antioxidant supplements have been considered a potential way to reduce cancer risk.

Now, scientists have discovered that certain immune cells depend on these molecules to activate and destroy cancer cells and that tumors exploit this dependency by releasing natural antioxidants to shut down the immune attack.

The findings, published today in Science, could help improve the effectiveness of cancer immunotherapies.