Among the many mysteries of autism, medical science has no clue about what causes 90% of the cases of this heartbreaking disease. Now, a new study suggests that the mothers of some autistic children may have made antibodies against their fetuses's brain tissue during pregnancy and that these antibodies crossed the placenta and caused brain damage that resulted in autism.

Affecting approximately 1 in 150 children in the U.S., autism causes a variety of problems. Autistic children have impaired social interactions, communication disorders and repetitive behaviors. Genetic, metabolic and environmental factors have all been implicated in various studies.

"Now our research suggests that the mother's immune system may be yet another factor or a trigger in those already predisposed," says lead investigator of the new study Harvey Singer, M.D., director of pediatric neurology at Johns Hopkins Children's.

The idea of immune system involvement in autism is not completely new. In the past, there was anecdotal evidence of unusual antibody levels in some autistic children and from postmortem brain tissue studies showing immune abnormalities.

Antibodies are proteins the body makes to combat infections such as viruses and bacteria. Sometimes, however, immune system disorders cause them to fight mistakenly against healthy tissues. The fact that the majority of children with autism have no evidence of autoimmune disease led researchers to ask if antibodies transferred from mother to child during pregnancy could interfere directly with fetal brain development.

To answer this question, researchers used a technique called immunoblotting, in which antibodies were taken from the blood and exposed to brain tissue to check whether the antibodies reacted against specific brain proteins.

Comparing the antibody-brain interaction in samples obtained from 100 mothers of autistic children and 100 mothers of children without autism, researchers found either stronger reactivity or more areas of reactivity between antibodies and brain proteins in about 40 percent of the samples obtained from the mothers of autistic children. Further, the presence of maternal antibodies was associated with developmental regression in children, that is, increasingly immature behaviors that are a classic symptom of autism.

While the findings suggest a clear association between autism and the presence of fetal brain antibodies, the researchers say that further studies are needed to prove that particular antibodies do indeed cross the placenta and damage the fetal brain.

"The mere fact that a pregnant woman has antibodies against the fetal brain doesn't mean she will have an autistic child, Singer says. "Autism is a complex condition and one that is likely caused by the interplay of immune, genetic and environmental factors."

The study is published in the February issue of the Journal of Neuroimmunology.