Under Your Skin

What happens above, within, and under your skin is a maelstrom of fascinating biological processes.


| May 2014



close-up of skin

A wide range of defenses keep grit and germs from under your skin.

Photo by Fotolia/Deyan Georgiev

Skin is of course your body's largest organ, but what are the functions of that organ? What happens within and under your skin? Leading dermatologist Dr. Robert Norman answers these questions in The Blue Man and Other Stories of the Skin (University of California Press, 2014), a compelling and accessible introduction to the life of your skin, while also recounting his experiences with patients and their memorable and mysterious skin maladies. Excerpted from "What Covers Us?" this selection goes into the basics of what's going in around and under your skin.

The finest clothing made is a person’s own skin, but, of course, society demands something more than this.
— Mark Twain

Our skin, at the most basic level, defines us.

The skin is the body’s largest organ, averaging twenty square feet and nine pounds; it makes up 16 percent of the body’s weight. The skin is complicated but amazing in structure; it can be the target and dwelling place of thousands of tiny viruses, bacteria, or yeasts, yet does a stellar job of living with the good and keeping out the bad, all to protect the inner environment.

Epidermis, Dermis, Subcutis: A Tour of the Skin

The epidermis, the outermost layer of skin, is a major part of the immune system. It is filled with Langerhans cells that form a first line of defense against environmental threats, identifying foreign materials and dangerous substances and ridding us of them. The epidermis is also part chemical plant, synthesizing vitamin D in the presence of sunlight, transforming a variety of helpful chemical compounds that interact with it, and inactivating substances that could be dangerous for us to absorb.

The epidermis protects the body from all kinds of insults. As Arthur and Loretta Balin write, “It seems fitting that the organ that defines the boundary between outside and inside worlds would be involved in the essential immune-system task of distinguishing between self and other.” Poison ivy is a common example: it is known to affect over 350,000 people in the United States annually, and demonstrates the wide range of possible sensitivities and reactions to exposure. Here’s how the reaction to poison ivy works. Urushiol is a chemical within the sap of the poison ivy plant that binds to the skin on contact. Urushiol shimmies its way into the skin and is broken down by T lymphocytes (or T-cells) that recognize it as a foreign substance, or antigen. The T-cells send out inflammatory signals called cytokines and the immune system pumps up the volume and calls in the troops—white blood cells. Still under the command of cytokines, the white blood cells turn into macrophages (super Pac Men) that eat up the foreign urushiol but also cause collateral damage to the normal tissue skin, resulting in inflammation and a dermatitis. A severe allergic reaction and blistering and oozing may occur in susceptible individuals; the fluid is produced by the body as blood vessels develop gaps and leak fluid through the skin. Approximately a quarter of the people exposed to poison ivy have no allergic reaction, while in extreme cases a anaphylactic reaction can occur. With age and repeated exposure, the sensitivity usually decreases.