Children with autism have inflammation in their brains, although it is not yet clear whether the inflammation actually causes the condition, researchers said on Monday.

Tests on the brain tissue of 11 patients with autism who had died and spinal fluid from six living children with autism showed the activation of immune system responses, the team at Johns Hopkins University School of Medicine in Baltimore and the University of Milan found.

“These findings reinforce the theory that immune activation in the brain is involved in autism, although it is not yet clear whether it is destructive or beneficial, or both, to the developing brain,” said Dr. Carlos Pardo-Villamizar of Johns Hopkins, who led the study.

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.

In a study published in the online edition of the Annals of Neurology, Pardo and colleagues said they found abnormal activity by immune system signaling chemicals called chemokines in the autistic patients.

“This ongoing inflammatory process was present in different areas of the brain and produced by cells known as microglia and astroglia,” said Pardo.

“Scientists have found hints that the immune system may be involved in autism, but not all studies have confirmed this,” Pardo added in a statement. “We wanted a more definitive answer, so rather than looking at the overall immune system, we focused on immune responses inside the relatively sealed environment of the nervous system.”

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

The condition is strongly influenced by genes. If one identical twin has autism, for instance, the other is also usually affected.

Pardo said more study would be needed to show if the inflammation itself underlies autism, or is a reaction to something else that causes the condition.