A vaccine that could ease asthmatic reactions has been described as "promising" by scientists.
A vaccine could reduce the need for medication
The discovery is being tested at the National Heart and Lung Institute at Imperial College London.
It works by "de-sensitising" asthmatics to the things that trigger their attacks, such as animal fur or dust.
This is done by injecting the patient with tiny amounts of the substance itself.
This technique has been known for some time, but exposing asthmatics to allergens - substances that trigger allergies - has always carried the risk of causing a serious attack.
The London researchers believe they have overcome this problem by modifying the basic ingredients of the vaccine - the protein allergens.
With this method, there is much less risk of side-effects
Dr Martyn Partridge, National Asthma Campaign
They have divided them into smaller units, which are still recognised by the immune system for the purposes of de-sensitisation, but are less likely to cause a massive attack.
Currently, asthmatics would have to undergo a series of injections, and protection would not be permanent, perhaps lasting only a few years.
But there are hopes that permanent protection could be achieved one day.
It is also not a cure - preliminary testing suggests that reactions would be reduced by half, although some asthmatics were left only with minor symptoms.
However, this would still potentially transform the lives of some severe asthmatics and save the NHS millions of pounds a year.
Asthma, in its most severe forms, is a life-threatening illness.
It is caused by an allergic reaction which causes the airways to swell up and close, making it difficult to breathe.
Modern drugs help to both relieve attacks and make them less likely to happen, but there is no cure.
The illness kills more than 1,500 people a year in the UK.
Dr Martyn Partidge, from the National Asthma Campaign, told the BBC that while it was "early days", the jab showed promise.
He said: "What you want to do is to prevent people developing the disease in the first place, or to switch it off if they already have it.
"With this method, there is much less risk of side-effects."