A new study out of Sweden appears to show a link between taking testosterone as part of gender-affirming care with changes in the immune systems of transgender men.As Scientific American reported, scientists studied a small group of 23 transgender men between the ages of 18 and 37 over the course of 12 months. Their goal was to see how taking testosterone would affect participants vulnerability to viral infections and autoimmune diseases, which have been shown to differ in men and women. As the outlet notes, men tend to be more vulnerable to viral infections, while women are more prone to autoimmune diseases, though it is unclear whether genetics, hormones, or other factors account for that difference. Related Senators ask Biden officials to remove barriers to testosterone access Testosterone is currently classified as a Schedule III controlled substance, which makes it harder to obtain. As part of the Swedish study, researchers took blood samples from all 23 trans men before they started gender-affirming hormone therapy. After three months on testosterone, a second blood sample showed that most of the participants testosterone levels were comparable to those of cisgender men and that their estradiol and progesterone levels had decreased. Stay connected to your community Connect with the issues and events that impact your community at home and beyond by subscribing to our newsletter. Subscribe to our Newsletter today A third blood sample taken after 12 months on testosterone showed a decrease in an immune response involving type I interferon, a protein the body uses to fight viral infections, and an increase in a signaling pathway involving tumor necrosis factor, which is involved in fighting bacterial infections through inflammation, according to Scientific American.Researchers also took blood samples from cisgender female donors and exposed them to either testosterone or an estrogen blocker. They found that testosterone appeared to be directly involved in an increase in the tumor necrosis factor response similar to what was shown in the trans mens blood samples after 12 months. They also found that testosterone also affected the type I interferon response but noted that previous studies have suggested that estrogen may also be involved.Essentially, the results of the study, published earlier this month in the journal Nature, suggest that taking testosterone may make people more vulnerable to viral infections while increasing their bodies ability to fight off bacterial infections. However, the studys authors urged caution. We cant say that these individuals are now more susceptible to any infection, and so on, but what we can say is that their immune profiles do alter or change in a way that is more similar to [cisgender] men, co-author Petter Brodin, a professor of pediatric immunology at the Karolinska Institute in Stockholm, told Scientific American.Dawn Newcomb, an assistant professor of medicine at Vanderbilt University Medical Center, who was not involved in the research, noted the studys small sample size and added that sex hormones are likely one mechanismnot the only mechanismat play.Nils Landegren, an assistant professor at Uppsala University in Sweden who co-authored the study, cautioned against drawing conclusions about the risks of gender-affirming hormone therapy for transgender men without further information about actual disease outcomes. We will need much larger numbers and longer follow-up, he said.We should just urge caution for people taking hormone therapies for any purpose, Brodin said, and make sure that we follow up to see that there are no health consequences that we dont intend.