As we age, our immune system slowly but steadily declines. But there’s a way to reverse that trend. A study published in the Aging Cell journal found that active cyclists in their 60s and 70s actually had immune systems just as strong as adults in their 20s. The reason: Exercise helps produce more T cells, which help fight off disease. Of course, a lot of factors determine overall health, but the researchers say that regular exercise can help serve as added protection against falling ill.