Autism researchers said that they had found a group of maternal antibodies that target proteins in the foetus' developing brain.
A study that compared 246 mothers of children with autism spectrum disorder (ASD) to 149 women with healthy children found that nearly a quarter in the first group had different combinations of these antibodies.
Antibodies are the foot soldiers of the immune system, latching onto viral or microbial intruders and tagging them for destruction by specialised "killer" cells.
But sometimes, for some unknown reason, these antibodies target our own, healthy proteins, becoming "auto-antibodies."
They play an important role in autoimmune diseases like lupus, rheumatoid arthritis and multiple sclerosis.
Just as pregnant women pass good antibodies on to their unborn children through the placenta, so too can they pass on malfunctioning ones which can target proteins the baby needs to develop, said study author Judy Van De Water, a professor of medicine at the University of California, Davis.
"We discovered that 23 percent of mothers whose children have autism have autoantibodies to certain proteins that are necessary for healthy neuron development," she told AFP by email of the study published in the journal Translational Psychiatry.
"These antibodies are not found in the blood of mothers (whose) children are typically developing."
ASD describes a broad range of impairments in which a person is unable or unwilling to communicate or interact with others, often cripplingly so.
Some patients have delays in cognitive development, whereas others can have dazzling gifts in specific fields such as maths or music. But the causes of the disease remain unclear.
ASD affects about one in 88 children in the United States.
Van De Water said the study revealed which seven proteins the autoantibodies latch on to, providing critical clues to the development of some forms of ASD, and possibly boosting the quest for an early, predictive test and treatment.
The researchers found 11 different combinations of seven proteins, each posing a different level of ASD risk.
"Very early behavioural intervention is effective in helping children with ASD improve their behaviours and abilities, so knowing this very early will be beneficial," Van De Water said.
"Because this test could be used prior to conception, the women could make a decision to use a surrogate or to prepare for having a child with ASD and undergoing early intervention."
One shortcoming of the study was that the samples from mothers were taken when their children were diagnosed, rather than during pregnancy, said the authors.
A separate paper published in the same journal pointed to a similar effect of human autoantibodies in rhesus monkeys.
Researchers took antibodies from women with ASD children, gave it to eight monkey females, and tested the effects on their offspring.
They found the monkey babies "displayed several behavioural differences, including inappropriate approaches to unfamiliar peers", said a press summary.
Male offspring had enlarged frontal brain lobes, consistent with brain scan results from autistic human children.