Your skin has a mind of its own, and it's a mind that doesn't always react well to stress. Your skin does its best to keep out viruses and other antigens (unwanted foreign substances), but it also has its own immune response when unwelcome guests manage to bypass the outermost layer. When this occurs, the skin facilitates the rush of white blood cells to the invasive substance, where they do their best to disable and destroy it before it advances any further.
Sometimes your skin goes overboard with enthusiasm, and the cells involved in the immune response become overactive, causing inflammation. When you get stressed out, your body orders more immune cells to the skin than are needed, and this can lead to blotchiness, redness and itching.
But stress wreaks havoc on your skin in other ways as well. When your body detects stress, it prepares you to run from a lion or to engage a rival caveman in battle. It does this by prompting the release of stress hormones from your adrenal glands. These hormones -- which include cortisol -- then trigger a number of other changes in your body. Since very little of our day-to-day stress involves lion attacks or caveman-on-caveman battles, many of these bodily changes aren't useful, and are even -- in the case of your skin -- unwelcome. Cortisol triggers glands in your hair follicles to crank up production of sebum, a useful oil that normally makes its way up and out of the hair follicle, taking with it dead skin cells. Produce too much, however, and the sebum can block up the follicle, leading to a pile-up of dead skin cells behind it, followed by inflammation and acne.
What's this have to do with exercise? Exercise makes you feel less stressed out for a reason: It lowers cortisol levels in the body (and often provides a pleasant endorphin boost to boot). Lower cortisol levels -- and thus lower stress -- means less chance of your skin overloading on immune cells, and more normal levels of sebum production.
But exercise benefits the skin in other ways as well, which we'll learn about in the next section.