New research from the University of Adelaide is the first in the world to explain why people with irritable bowel syndrome - also known as IBS - may not get relief from pain medications.
IBS is prevalent in up to 10% of people, and sufferers endure reduced quality of life through unexplained gut pain.
The study shows that pain-blocking cells in the bowels of IBS sufferers are abnormal.
"The gut contains specialised immune cells, known as monocytes and macrophages," said lead author Dr Patrick Hughes, NHMRC Peter Doherty Fellow with the University's School of Medicine.
"Our research has shown that in healthy people, these immune cells normally secrete opioid chemicals, like morphine, that block pain. But in people with IBS, the opioid production by these cells is defective," he said.
The research is a positive step in the battle to understand IBS and its clinical manifestations.
"This study is the first to give us a real understanding of the interaction between the immune system and pain symptoms in IBS patients," said Dr Hughes.
"If the immune system is defective, it's no wonder that people with IBS are experiencing ongoing periods of unexplained pain."
"It may also mean that painkilling medications taken by the patient to relieve their symptoms are not being adequately converted to pain relief," he said.
Although the exact cause of IBS remains unknown, Dr Hughes is confident his research will contribute to understanding and managing pain due to the condition.
"We have now confirmed the important role of the immune system in this pain response," he said.
"We hope that this work could eventually lead to more targeted treatments for IBS sufferers, to help treat or prevent the long-term pain they experience."
The research involved looking at samples from more than 100 people, half of them healthy and half suffering from IBS. The results are published in the journal Brain, Behavior, and Immunity.
The research was funded by the National Health and Medical Research Council.