HealthDay Reporter FRIDAY, May 6, 2016 (HealthDay News) -- The terrible birth defects caused by Zika virus appear to be the result of an immune system response that triggers prenatal brain cell suicide and obstructs fetal brain development, a new lab study reports. The virus apparently activates an immune receptor called TLR3, which the body uses to identify and defend against invading viruses, said lead researcher Tariq Rana. He is a professor of pediatrics and genetics at the University of California, San Diego. The hyper-activated TLR3 then proceeds to turn off genes that fetal stem cells need to specialize into brain cells, and appears to switch on genes that trigger cell suicide, Rana said. "We all have an innate immune system that evolved specifically to fight off viruses, but here the virus turns that very same defense mechanism against us," Rana explained. "By activating TLR3, the Zika virus blocks genes that tell stem cells to develop into the various parts of the brain." These findings help explain how Zika causes microcephaly , an abnormally small brain and skull development, and other neonatal brain-related birth defects, Rana said. They also show why there's no similar effect in adults, since their brains are already fully developed. There's also good news for the potential prevention of brain birth defects due to Zika. Rana and his team successfully blocked this process in laboratory stem cell samples by using a chemical that inhibits the action of TLR3. A drug that safely blocks TLR3 could possibly save babies from birth defects if given to Zika- infected women during the early stages of pregnancy , he said. "If some mom comes into the clinic and the doctor thinks there is a Zika infection established, you could give this med, and once the first trimester is done, you can take them off the med," Rana suggested. Zika is the first mosquito -borne virus ever proven to cause birth defects, according to the U.S. Centers for Disease Control and Prevention. To figure out why this happens, Rana and his team used human embryonic stem cells to create a laboratory model of a first-trimester human brain, also called a "cerebral organoid." The model's stem cells were designed to specialize and grow into the various different cells in the brain, much the same as they do during a fetus ' first trimester.