Scientists say they have uncovered new evidence which it is hoped could eventually help diagnose and prevent type 1 diabetes.
Professor Andy Sewell of Cardiff University, working with King's College London, witnessed human T-cells - which protect against disease - inadvertently destroying insulin-producing cells.
Insulin controls blood sugar levels and a lack of it can be fatal.
More than 160,000 people in Wales have been diagnosed with diabetes.
Prof Sewell, an expert in human T-cells from Cardiff University's School of Medicine, worked alongside diabetes experts from King's College.
Scientists isolated a T-cell from a patient with type 1 diabetes to view a process which results in the killing of insulin-producing cells in the pancreas.
It is the first time the process has been witnessed.
"This is a breakthrough and tells us where we can now focus in the future," said Professor Sewell, speaking on BBC Radio Wales.
He said people can have type 1 diabetes long before being diagnosed so, potentially in the future, at risk groups could be identified and then treated earlier.
"Type 1 diabetes is a result of the body's own immune system attacking and destroying the cells in the pancreas that manufacture the hormone insulin," he said.
"Insulin controls blood sugar levels and a lack of insulin is fatal if untreated.
"The mechanism by which the body attacks its own insulin producing cells in the pancreas is not fully understood.
"Our findings show how killer T-cells might play an important role in autoimmune diseases like diabetes and we've secured the first ever glimpse of the mechanism by which killer T-cells can attack our own body cells to cause disease."
Professor Mark Peakman from the National Institute for Health Research (NIHR) Biomedical Research Centre at King's College, London, said: "This first sight of how killer T-cells make contact with the cells that make insulin is very enlightening, and increases our understanding of how type 1 diabetes may arise.
"This knowledge will be used in the future to help us predict who might get the disease and also to develop new approaches to prevent it.
"Our aim is to catch the disease early before too many insulin-producing cells have been damaged."