How Your Belly Fat Is Linked to Your Immune System

You may not love your belly fat, but one large sheet of fat that stretches across your abdomen serves an important purpose: This fat, called the omentum, plays a role in the immune system, according to a new review.

No matter your weight, everybody has an omentum. The fatty structure connects many organs in the abdomen, including the spleen, the stomach and the colon. It resembles "an apron that hangs in front of the abdominal organs," according to the review, published June 1 in the journal Trends in Immunology.

But the omentum has other cells besides fat cells: Dispersed throughout the sheet are clumps of immune cells. Researchers discovered these clumps in rabbits in 1874, and named them "milky spots" because of their white appearance among the yellow fat cells. [11 Surprising Facts About the Immune System]

Later, researchers learned that these milky spots filter fluid, collecting information about what's going on in the abdomen, according to the review.

"The fluid around the abdominal organs doesn't just sit there," Troy Randall, a clinical immunologist at the University of Alabama at Birmingham and the senior author of the review, said in a statement. Instead, "It circulates through the milky spots."

These spots "collect cells, antigens and bacteria before deciding what's going to happen immunologically," Randall said. Then, they can launch an immune response.

These abilities — to monitor what's going on in the abdomen and respond — prompted a British surgeon to refer to the organ as "the policeman of the abdomen" in the early 1900s, according to the review. In particular, the surgeon had noticed that the omentum played a role in reducing inflammation in a thin membrane that lines the abdominal cavity, called the peritoneum, and also helped heal surgical wounds.

But the omentum's functions can also be detrimental to the body, especially when it comes to how the structure interacts with cancer cells.

Cancer of the omentum is rare, but other types of cancer in the body can spread to the omentum, according to the review. Indeed, the omentum is the most common site to which gastrointestinal and ovarian cancer spread, in a process called metastasis.

Cancer cells from other organs can reach the omentum through the abdominal fluid. Like other cells, these cancer cells are filtered through the milky spots. But instead of launching an immune response against the cancer cells, the omentum traps and ultimately protects them.

"The omentum makes the wrong decision" when it comes to tumor cells, Randall said. "It decides to provide tolerance instead of immunity."

Stuck in the omentum, these tumor cells can grow and multiply, according to the review.

Scientists hope that by learning more about the omentum and the role it plays in tumor metastasis, they can develop new ways to treat these cancers.

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

Originally published on Live Science.