Children with autism show different immune system responses from children without the condition, and these might be measured in the blood for a possible screening test, U.S. researchers reported.

Two studies presented to a conference on autism help support other research that suggests subtle differences in the immune function of children with autism.

Autism is a brain disorder usually seen as children become toddlers. Affecting an estimated two to five out of every 1,000 children, autism has a spectrum of symptoms that include difficulty with social interaction and repetitive behaviors.

No one knows what causes autism, although experts have largely rejected purported links with childhood vaccines.

Scientists at the 4th International Meeting for Autism Research in Boston presented studies looking at the blood of children with autism.

Judy Van de Water of the University of California, Davis, and colleagues separated immune cells from 30 children with autism and 26 non-autistic children aged 2 to 5. They mixed in toxins and bacteria.

In response to bacteria, the researchers saw lower levels of immune signaling proteins called cytokines in the group with autism. These children also had irregular responses to a plant protein, but not to other toxins or to a measles, mumps and rubella vaccine.

"Understanding the biology of autism is crucial to developing better ways to diagnose and treat it," Van de Water said in a statement.

A second team at the same center took blood samples from 70 children aged 4 to 6 with autism and from 35 other children.

The children with autism had 20 percent more immune system cells called B cells and 40 percent more natural killer cells.

There also seemed to be differences in other proteins in the blood, although the researchers are still sifting through the data.

"From these results we think it is highly likely that there are differences we can detect in blood samples that will be predictive of the disorder, though we are still some years away from having an actual diagnostic blood test for autism," said researcher David Amaral, who led the study.

What good would this do, as there is no cure?

"There is a growing view among experts that not all children with autism are 'doomed to autism' at birth," Amaral said in a statement.

"It may be that some children have a vulnerability, such as a genetic abnormality, and that something they encounter after being born, perhaps in their environment, triggers the disorder," he added.

"Studying the biological signs of autism could lead to new ways to prevent the disorder from ever occurring. And even if it can't be prevented, intervening early in life -- ideally shortly after birth -- could greatly improve the lifetime outlook for children with autism."