Pop star Selena Gomez revealed Thursday she underwent a kidney transplant as part of her treatment for lupus. Gomez posted an Instagram showing herself in a hospital bed beside actress Francia Raisa, who donated her kidney.

“I’m very aware some of my fans had noticed I was laying low for part of the summer and questioning why I wasn’t promoting my new music, which I was extremely proud of,” Gomez wrote. “So I found out I needed to get a kidney transplant due to my Lupus and was recovering.”

In the Instagram post, Gomez noted that lupus “continues to be very misunderstood.” An autoimmune disease, lupus is a chronic condition in which the immune system attacks healthy cells and tissues. Damage can occur to various parts of the body, including joints, skin, kidneys, the heart, the lungs, blood vessels and the brain.

Lupus mostly affects women of childbearing age, though men and children can also develop the disease. It’s estimated that at least 1.5 million people in the United States have lupus, according to the National Resource Center on Lupus.

Symptoms of lupus vary but can include extreme fatigue, arthritis, fever, skin rashes and kidney problems. When lupus affects kidneys, it can often inflame them to the point where they’re unable to get rid of waste products and function effectively. Lupus affecting the kidneys typically requires “intensive drug treatment to prevent permanent damage,” according to the Alliance for Lupus Research. While there is currently no cure for lupus, the disease can be dealt with through treatment.

Gomez declined to say on her Instagram when or where she underwent the kidney transplant, but said it was what she “needed to do” for her health. She also posted a photograph of her scar from the transplant and promised to reveal further details about her condition as time passed.

“I want to publicly thank my family and incredible team of doctors for everything they have done for me prior to and post-surgery,” Gomez wrote. “And finally, there aren’t enough words to describe how I can possibly thank my beautiful friend Francia Raisa. She gave me the ultimate gift and sacrifice by donating her kidney to me. I am incredibly blessed.”