Your Abdomen's Layer of Fat Is Also an Important Immune Organ

The omentum is a large sheet of fat that stretches across the gut, and researchers believe it may play an important role in the immune response to certain tumors.

Scientists believe that the omentum — a little known organ considered the “policeman of the abdomen” – may have answers about how to halt the growth of aggressive tumors.

The omentum lies like an apron over the peritoneal, or abdominal, cavity — a large sheet of fat that stretches across the intestines, liver, and stomach. The function of the organ itself is poorly understood, but researchers at the University of Alabama at Birmingham believe that it may play an important role in the immune response to certain tumors.

In a review published today in journal Trends in Immunology, scientists look at the role of so-called milky spots — clusters of white blood cells that speckle the omentum. They circulate the liquid in the abdominal cavity and much like lymph nodes, they filter and identify foreign antigens in the body, deciding whether or not to trigger an immune response.

While they are considered a first line of defense against foreign toxins and infections, as Tony Randall, a professor of medicine at the university and co-author of the review explained, these milky spots don’t always make the right call. 

“They make a good immune response to bacteria,” Randall said. In this case, the milky spots cause the omentum to release inflammatory molecules to protect the body. “But with tumor cells, rather than triggering a robust immune response, it kind of turns off,” he added. “The process has kind of gone wrong.”

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It is an immunological puzzle that Randall and co-author Selene Meza-Perez want to better understand. If they can explain why the omentum is tolerating the grown of tumor cells within these milky-spots, perhaps they can identify the mechanisms that would trigger an immune response, halting the growth of these tumors.

Their suspicion is that the omentum is somehow connected to the mucosal immune system that has evolved to recognize foreign antigens in the gut. Because we introduce a wide variety of food from different plants and animals into our body when we eat, the gut has evolved to tolerate certain foreign substances. 

“The gut identifies these different microbial cells, etc., and it is tolerant,” Randall said. “It doesn’t make an inflammatory response, because most of the stuff in our gut is innocuous, not a pathogen.”

While this function is helpful in the gut — an inflammatory response to foreign bodies in the stomach would be quite painful, Randall explained — it is detrimental in the omentum. With the immune system disengaged, the milky spots become a breeding ground for aggressive tumors, such as such as ovarian and gastrointestinal cancer.

And because the omentum is 99 percent fat, the tumor draws considerable energy from the organ, allowing cells to quickly multiply.

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If they can understand the mechanism these milky spots use to warn the immune system of an imminent threat, Randall said they might be able to “retrain the immune system to say, in fact, there is something dangerous happening here.”  One way to do that may be by tricking it into believing there is an infection.

But first scientists need to better understand how the omentum is connected to the gut and how the cells in milky spots are making decisions about perceived threats. In short, Randall said, they need to better understand the “decision points of cells.”

“Then,” he said, “hopefully we can intervene to promote the decision we would like.”

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