Ovarian cancer is the deadliest of all female reproductive cancers and claimed the lives of an estimated fifteen-thousand American women last year. The disease is treated most effectively when it’s discovered before it spreads to other organs, but most of the time that’s not the case.

Now, new work sheds light on why ovarian cancer is often referred to as the silent killer: In a deadly game of hide-and-seek, ovarian tumors release fatty substances that help them evade the immune system.

Researchers at Johns Hopkins University made this discovery by sampling tumor secretions from twenty-five women with ovarian cancer. The scientists exposed these secretions to immune cells in the lab and found that certain fatty substances in the secretions… known as lipids… almost completely blocked the immune cells’ ability to activate the body’s early warning system, allowing the cancer to spread unchecked throughout the body. The study was published in a recent issue of the journal Clinical Cancer Research.

The study may help explain why fewer than twenty percent of women discover the cancer before it spreads: The tumor secretions targeted immune cells within a mere four hours in a laboratory experiment, and blocked the immune cells’ action for an extended period of time.

The results have encouraged researchers to look at other lipids in the body in hopes of exploring how they influence the outcomes of other diseases. Now, thanks to their efforts, ovarian cancer has one less place to hide.