A study published in Arthritis & Rheumatology provides evidence that the prevalence of autoimmunity—when the immune system goes awry and attacks the body itself—has increased in the United States in recent years.

For the study, researchers looked for antinuclear antibodies (ANA)—the most common marker of autoimmunity—in the blood of 14,211 participants from the US National Health and Nutrition Examination Survey, with approximately one-third from each of three time periods: 1988–1991, 1999–2004, and 2011–2012.

The prevalence of ANA was 11.0% in 1988–1991, 11.5% in 1999–2004, and 15.9% in 2011–2012, which corresponds to 22, 27, and 41 million affected individuals, respectively.

“The reasons for the increasing prevalence of ANA, which were most pronounced in adolescents, males, and non-Hispanic whites, remain unclear,” said senior author Frederick W. Miller, MD, PhD, of the National Institute of Environmental Health Sciences. “They are concerning, however, as they may herald an increase in autoimmune disorders, and emphasize the need for additional studies to determine the driving forces underlying these findings and to enable the development of possible preventative measures.”