Some abdominal fat is bad, but some is very good - a layer deep inside next to your organs called the omentum has an important immune function.

You may not like thinking about your belly fat, but a layer of fatty tissue that hangs over your stomach is performing a remarkable job: protecting your body from infection by regulating the immune system.

The omentum is an apron-like structure that hangs down from the stomach and protects the organs of the abdomen, including the liver, stomach and intestines. One of its jobs is storing fat, but among biologists it has earned the nickname 'policeman of the abdomen', for its role in regulating the immune system. It is not very well known, but in humans it is very large, covering an area of 1,500 square centimetres.

This organ is different from fatty tissues elsewhere in the body, and it's not the same as the fat stored directly under the skin. The omentum has a series of small white dots on its surfaces exposed to the abdominal cavity. These 'milky spots' are the site of the omentum's crucial activity in immunity.

"The fluid around the abdominal organs doesn't just sit there, it circulates through the milky spots," said Troy Randall of the University of Alabama at Birmingham, author of a review on the role of the omentum published in Trends in Immunology. "Milky spots collect cells, antigens, and bacteria before deciding what's going to happen immunologically."

The organ itself isn't a static structure. It moves around the abdominal cavity to sites of damage.

"The omentum was noted to move about the peritoneal cavity and occlude sites of inflammation, such as ruptured ovaries, inflamed appendices, ulcerated intestines, or wounds due to trauma or surgery," the authors write in the paper.

When it gets to these sites, this strip of fat helps to kill potential infections and promote wound healing. Despite this, the omentum is "poorly understood and often overlooked" in its important role, the study authors write.

How the omentum fits around the other organs of the abdomen. Olek Remesz / Wikimedia Commons

Links to cancers

This doesn't mean that having more belly fat makes you healthier. Having a lot of excess fat has strong links to higher risks of heart disease, diabetes and stroke.

This underappreciated organ is beginning to be recognised as a promising target for developing new cancer treatments. The milky spots are a problem area when it comes to cancerous cells, as they can't distinguish between healthy and unhealthy cells.

"In concerns to tumour cells, the omentum makes the wrong decision," Randall said. "It decides to provide tolerance instead of immunity."

Cancerous cells tend to accumulate at the milky spots, promoting mestastasis.

"If we can figure this out, then we can start really making inroads on cancer treatments because, in most cases, you don't even catch ovarian cancer until it metastasizes," Randall said.

"Understanding how cancer changes the immune system will lead us directly to ways to intervene and, hopefully, start to turn things around."