The disease is caused by the body destroying cells in the pancreas that control blood sugar levels.
The immunotherapy - tested on 27 people in the UK - also showed signs of slowing the disease, but this needs confirming in larger trials.
Experts said the advance could one day free people from daily injections.
Aleix Rowlandson, from Lancashire, was diagnosed in 2015 aged 18.
"Your blood sugars affect how much energy you have," she told the BBC.
"If they're high, they can make you feel tired. If they're low, you can feel shaky.
"I'm more optimistic knowing that the study has gone well and they can use that to find further treatments.
"Even if it doesn't help me, myself, and it might help other people in the future, I'm very happy."
Aleix's immune system is attacking her beta cells, which release the hormone insulin to keep blood sugar levels stable.
As a result, she has to inject insulin several times a day.
Aleix is taking part in the trials of immunotherapy at the National Institute for Health Research Biomedical Research Centre at Guy's and St Thomas'.
It is an attempt to stop her diabetes by tapping into the immune system's natural checks and balances.
The body's defence system is primed to attack hostile invaders.
But it also has "regulatory T cells", which calm the immune response and prevent it attacking the body's own tissues.
Immunotherapies try to get regulatory T cells on-side by exposing them to fragments of proteins found in beta cells.
Prof Mark Peakman, from King's College London, told the BBC News website: "This is a landmark in the sense it's the first time it has been done.
"Importantly, [the trial] shows the overall safety is good and there is some evidence we're restoring the balance and getting some regulatory T cells activated."
Patients given the therapy did not need to increase their dose of insulin during the trial.
However, it is too soon to say this therapy stops type 1 diabetes and larger clinical trials will be needed.
And further types of immunotherapy that should deliver an even stronger reaction are already underway.
Beta cell saver
The trial focused on patients newly diagnosed with type 1 as they still have about a fifth of their beta cells left.
Even retaining these cells would make it easier to manage the condition, but the ultimate goal is to intervene even earlier to hopefully prevent the disease starting.
However, it is not likely to help people diagnosed with type 1 years ago.
Prof Peakman added: "At that stage, most of the beta cells have gone and we don't find, with any therapies tried, any evidence of regeneration so it seems unlikely to help someone who has had the disease for a while."
All the volunteers were injected either every two or four weeks for six months.
Karen Addington, the UK chief executive of the type 1 diabetes charity JDRF, said: "Exciting immunotherapy research like this increases the likelihood that one day insulin-producing cells can be protected and preserved.
"That would mean people at risk of type 1 diabetes might one day need to take less insulin, and perhaps see a future where no-one would ever face daily injections to stay alive."
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.
About 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 tends to develop later in life and is linked to lifestyle and being overweight.