Allergic reactions are triggered when the immune system mistakes a normally harmless substance for a dangerous invader and launches an antibody attack in response.

Antibodies are designed to attack one substance and one substance only and different antibodies have different effects on the body - leading doctors to expect a different reaction to a different allergy.

However, some people experience similar reactions even when they are exposed to different allergens, such as pollen and dust mites.

The reason for this may lie in research that undermines doctors' current understanding of how the immune system works.

Foot soldiers

An immune response kicks off when white blood cells known as B lymphocytes get a message that a potentially dangerous substance has entered the body.

They then produce antibodies, which are Y-shaped proteins, which attach themselves to the intruding substance - and carry them away to be destroyed.

They do this by using molecular "tweezers" at the tip of each arm - what is known as the antigen-binding site, because they attach themselves to particles called antigens on the offending substance.

Scientists always thought that the two antigen-binding sites were identical, so any one antibody would only be able to tackle one substance.

But the new study, carried out by scientists in Amsterdam, found that some antibodies have different antigen-binding sites - so one arm will bind to antigens on pollen while the other will attach itself to dust mites.

Strange behaviour

The study focussed on immunoglobulin G4 (IgG4) antibodies, which are involved in allergic reactions.

Rob Aalberse and his colleagues at the University of Amsterdam started investigating the idea after they noticed the antibodies behaving strangely.

When mixed with their target antigen, the antibodies only picked up one molecule. If both arms were identical, the antibody should pick up two at a time.

To find out what was going on, they stuck pollen antigens to the inside of a test tube and then rinsed it out with pollen antibodies. Next, they washed the tube with dust mite antigens.

They found that the dust mite antigens stuck to the side of the tube, showing that the antibodies had bound to both antigens at the same time.

Since the study was published, they have found antibodies that bind to both banana and cat dander antigens, and to diptheria and tetanus toxins.

'Intriguing'

Greg Winter, an antibody engineer at the Medical Research Council's Laboratory of Molecular Biology in Cambridge, said the research was intriguing but more work would need to be done before any firm conclusions could be drawn.

"Once you get allergic to one thing, you can develop a whole set of allergies," he said.

"But what the researchers were unable to determine is how the hell this is happening."

One possibility was that the double-action antibodies developed from normal antibodies that break up and then re-form with another type.