Scientists have produced compelling evidence that autism may in some cases be linked to inflammation of the brain.
Autism is a mysterious condition
They found certain immune system components that promote inflammation are consistently activated in people with autism.
Autistic children have difficulties in social interaction, may show repetitive behaviours and may have unusual attachments to objects or routines.
The Johns Hopkins University research is published in Annals of Neurology.
Autism is a disorder of the developing brain that appears in early childhood. It is estimated to afflict between two and five of every 1,000 children and is four times more likely to strike boys than girls.
The condition has a strong genetic component. For instance, in identical twins, where one twin has autism, both are usually affected.
However, the number of children with autism appears to be increasing more than expected for a genetic disorder.
This suggests too that genetic abnormalities require the influence of other factors to cause the disorder.
Birth complications, toxins, diet, and viruses and other pathogens have been suggested, though there is no strong evidence for any of these.
In recent years, there have been scientific hints of immune system irregularities in children with autism, but not all studies have confirmed this.
The Johns Hopkins team sought a more definitive answer by looking not at the immune system overall, but at immune components inside the relatively sealed environment of the nervous system.
They examined brain tissue from 11 people with autism, aged five to 44 years, who had died of accidents or injuries.
Compared with normal control brains, the brains of the people with autism were found to contain abnormal patterns of immune system proteins called cytokines and chemokines consistent with inflammation.
Researcher Dr Carlos Pardo-Villamizar said: "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."
Similarly, samples of cerebrospinal fluid obtained from six children with autism were also found to contain elevated levels of cytokines.
The researchers say it might eventually be possible to develop a diagnostic test for autism based on looking for signs of inflammation - and that treating this inflammation might reduce the symptoms of autism.
However, Dr Andrew Zimmerman, a paediatric neurologist at the Kennedy-Krieger Institute in Baltimore who also worked on the study, said it was possible that inflammation was produced as a result of the brain trying to combat some other process damaging to brain cells.
A spokesperson for the National Autistic Society said other scientists had also examined the possible connection between the immune system and autism.
One study has linked the condition to the disease encephalitis, while another found raised levels of nitric oxide in the plasma of children with autism.
This chemical plays a role in the immune response, and which is known to affect neurodevelopmental processes.