Japanese immunologist Shimon Sakaguchi was awarded the Nobel Prize in physiology or medicine on Monday along with two other scientists for their discovery concerning peripheral immune tolerance.

Sakaguchi, 74, a professor at University of Osaka, discovered a new class of T cells called regulatory T cells, which prevent immune cells from attacking our own body.

“Sakaguchi was swimming against the tide in 1995, when he made a key discovery,” the Nobel Committee said in a tweet. “At the time, many researchers were convinced that immune tolerance only developed due to potentially harmful immune cells being eliminated in the thymus, through a process called central tolerance.