ADHD can cause serious problems
Children with attention deficit hyperactivity disorder (ADHD) tend to be hyperactive, impulsive and have difficulty concentrating.
Some experts believe ADHD may affect up to one in ten of all children, although this is hotly disputed by others who believe doctors have been too quick to use the condition as a convenient way to explain away behavioural difficulties.
The causes of the condition are unknown. One theory is that it is linked to an imbalance of brain chemicals such as dopamine and norepinepherine.
Drugs such as Ritalin, which boost the activity of these chemicals, have successfully reduced ADHD symptoms.
Key chemical
Now new research suggests a chemical called interleukin 1 may play a vital role.
Interleukin 1 helps to regulate the immune system, but also appears to play a role in helping to keep brain cells in good condition, and to aid their development before birth.
Researchers from Israel tested the theory that an imbalance of interleukin 1 leads to problems in the brain cells that respond to chemicals such as dopamine and norepinepherine.
They found that children carrying a genetic variation which upsets the function of interleukin, were at greater risk of developing ADHD.
Researcher Dr Ronnen Segman, of Hadassah Hebrew University Medical Center, Jerusalem, told BBC News Online that the study suggested that immune system chemicals may play a role in a range of psychiatric disorders.
He said: "This extends our understanding on the causes of such disorders, and may form a basis for future novel preventive or interventive opportunities."
The research is published in the journal Molecular Psychiatry.
Professor Julio Licinio, editor of the journal, told BBC News Online: "This is in my view, the first report of a possible immune dysfunction in ADHD.
"If confirmed these findings will lead to new research on treatment strategies targeted to correct such an immune defect.
"ADHD is a serious health problem in children, affecting not only school performance but social relations and family dynamics.
"Current treatments are not always effective, and the search for new approaches is critically needed."