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The team at the University of Lincoln say the findings could help develop new ways to prevent and treat the disease.
Diabetes UK said the findings were "impressive".
In type 1 diabetes, the immune system destroys the beta cells that make insulin - the hormone needed to keep blood sugar levels under control.
Studies looking at the unique antibodies made by patients with type 1 showed there were five key targets that the immune system attacked.
But working out exactly what they were has been like identifying someone from their silhouette.
Studies long ago discovered some of the targets, but the final one has proved elusive for two decades.
Dr Michael Christie, who led the research at the University of Lincoln, told the BBC: "With this new discovery, we have now finished identifying what the immune system is targeting - we have the complete picture."
The targets are:
And the final piece of the puzzle, tetraspanin-7
The more technically named ones are largely involved in secreting or storing the hormone insulin.
But Dr Christie says having the complete picture could help transform care for type 1 patients.
He said: "Once the immune system decides it wants to get rid of something it's very hard to stop, so diabetes has proved to be a difficult disease to prevent.
"So we're hoping that, by having identified the major targets in the disease, we can find ways to prevent it by blocking the immune response to these five proteins without leaving that person vulnerable to infections.
"With recent improvements in our understanding of the disease I'm very hopeful we'll develop a treatment now; I have a lot more confidence than even five years ago."
There are two main types of diabetes -
Type 1 - where the pancreas does not produce any insulin
Type 2 - where the pancreas does not produce enough insulin - or the body's cells do not react to insulin
Type 1 diabetes can develop at any age, but usually appears before the age of 40, particularly in childhood.
Around 10% of all diabetes is type 1, but it is the most common type of childhood diabetes, so it is sometimes called juvenile diabetes or early-onset diabetes
Type 2 diabetes is largely caused by poor lifestyle. Around 90% of adults with diabetes have type 2, and it tends to develop later in life than type 1.
Source: NHS Choices
Dr Emily Burns, from the charity Diabetes UK, said: "In order to prevent type 1 diabetes, we need to fully understand how the immune response that damages insulin-producing cells develops in the first place.
"Dr Christie's impressive research is helping us to do just that.
"We hope that the findings here will be used to improve the identification of those at risk of type 1 diabetes and, in the long term, inform the crucial development of therapies