Johns Hopkins University research, published in Annals of Neurology, "produced compelling evidence that autism may in some cases be linked to inflammation of the brain."
They found certain immune system components that promote inflammation are consistently activated in people with autism.
The condition has a strong genetic component [..., but] the number of children with autism appears to be increasing more than expected for a genetic disorder.
This suggests ... 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.
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 [...]."
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.
Another study found raised levels of nitric oxide in the plasma of children with autism. Nitric oxide plays a role in the immune response, and is known to affect neurodevelopmental processes.