( CNN ) Could a simple blood test someday tell if you 're genetically predisposed to post-traumatic stress disorder ? That 's what a team of international researchers is hoping after finding a genetic marker linked to PTSD in the blood samples of Marines stationed in conflict zones . `` We 'll draw the blood and have a way to do this very rapidly and start to tease apart who is a little more at risk and who is a little more resilient for PTSD , '' says principal investigator Dr. Dewleen Baker of the University of California-San Diego . `` It 's exciting . '' PTSD can occur after many types of trauma : rape , torture , child abuse , natural disasters and car , plane , and train wrecks , to name a few . According to the PTSD Alliance , more than 13 million Americans have PTSD and the societal cost is in the billions . Women are about twice as likely as men to develop the disorder . Symptoms tend to cluster into three areas : . •Reliving the event via nightmares or vivid images , along with an extreme reaction such as uncontrollable shaking , chills or heart palpitations . •Avoiding reminders of the event , including becoming emotionally withdrawn and detached from friends , family and everyday activities . •Being hyperaroused , easily startled , irritable , angry , or having difficulty sleeping or concentrating . As one might expect , service members are hardest hit . The Department of Veterans Affairs estimates 30 % of all Vietnam vets have experienced PTSD . Among troops recently deployed to Iraq and Afghanistan , PTSD can run as high as 20 % in any given year . But why does severe emotional trauma trigger PTSD in some people and not others ? The idea that your genes play a role in whether you develop PTSD is a popular focus of recent research . Scientists have discovered genes that help regulate fear reactions in mice . The lack of a fear-regulating brain chemical called gastrin-releasing peptide led to greater fear response among the rodents . In another study , mice without a protein necessary to form `` fear memories '' were less likely to freeze up and more willing to explore unknown spaces . Studies of twins show heredity accounts for about 30 % of the differences in response to trauma , with identical twins much more likely to both develop PTSD than fraternal twins . Other research has looked into the role of inherited brain differences , mental disorders , or addictive tendencies . An unusual avenue of research is how our immune systems may contribute to the development of PTSD symptoms . Prior studies of people diagnosed with PTSD compared to control groups without the disorder suggest differences in genes related to inflammation may play a role . `` The body is built to keep us alive and it functions as one big system , '' says Baker , discussing the interface between stress and immune chemicals . `` The systems all talk to each other . '' The recent study , published in the journal Molecular Psychiatry , is unique because it compares blood samples from 188 Marines taken before and after they were deployed to combat zones . Another group of 96 Marines was also studied to verify the results . `` Under this experimental design , not only can we identify differences between U.S. Marines with PTSD and without , but we can go back in time , so to speak , to see if any of the Marines who eventually developed PTSD '' showed evidence that it would emerge , says the study 's co-senior author , Christopher Woelk . `` In this vein , we are able to start labeling findings as being putatively 'causal ' in nature . '' In the blood of Marines ultimately diagnosed with PTSD , the study found biomarkers associated with gene networks that regulate innate immune function -- the body 's first line of defense -- and interferon , a protein responsible for kicking the immune system into action . In addition , this hypervigilant immune response occurred before and after exposure to trauma . `` Why do certain people start out with slightly higher immune response and others have less ? Is it somehow just built in or have they been exposed to some kind of pathogen ? '' asks Baker . Instead of pathogens , another possible explanation is that the inflammatory response is activated by the stress of going off to war . `` You could try to dampen down the activation and see if that improves symptoms or prevents development of PTSD , '' says Baker . Baker stresses these questions are purely hypothetical at this time and that the study needs to be replicated and expanded . `` We 're early in the process of having some clues as to what might predict risk and resilience , and with more research we 'll begin to have effective preventions and treatments , '' says Baker .
More than 13 million Americans have PTSD . Comment: Immune system plays a role in risk and resilience .