Healthy skin is a first responder. It helps keep intruders - from UV rays to bacteria - from hacking into the system.

And it's amazingly up to the task. The largest organ in the body, skin protects our insides, regulates heat and cold, manufactures hormones, is self-repairing, creates new cells and eliminates dead cells, hosts many of the body's other components (like hair and sweat glands) - and through it all, stays firm, supple, and strong.

The Stats Skin is the largest and heaviest organ in the human body, with a surface area of approximately two square meters, and weighing four to five kg. That accounts for 16% of bodyweight.

On your eyelids, skin is 0.5mm thick, and it gets to 4mm thick on the palms of the hands and soles of the feet. An integumentary system, skin allows gasses to diffuse through its interstitial fluid. (Interstitial fluid is a solution that surrounds all cells.)

Skin is also a smart manufacturing plant. As an endocrine organ, it makes hormones (including a thyroid hormone), Vitamin D, some steroids, and melanin. It also creates some of the neurotransmitters and hormones that are found in the brain.

And did you know that freckles and moles are simply patches of skin that contain more melanin that surrounding skin? Melanin is produced by the skin's melanocytes, and protects the skin by absorbing UV rays.

The Layers Between you and the world, there's the epidermis. Here the cells are tightly packed together, yet the entire layer is thinner than plastic wrap. It's stain-resistant and waterproof, holding moisture in and keeping unwanted water out.

Cells at the bottom of the epidermis are constantly dividing, and pushing the new cells up to the surface. There the new cells die and flake off. As they die, they are filled with keratin - a tough protein that also acts as a protector. In fact, we each have a completely new epidermis about every 30 days.

The next layer down is connective tissue called the dermis. Because it contains collagen and elastic fibers, the dermis brings support and firmness to the skin. It's also the layer that is vulnerable to wrinkling.

The dermis is where we find nourishing blood cells, nerve tissue for feeling, and muscle tissue. When you get "goosebumps", this is the layer that's causing it. It also contains hair follicles, sweat glands, and oil glands. Oil glands, aka sebaceous glands, manufacture a lubricant for skin and hair called sebum.

Woven through all this are blood vessels. If you need to cool down, the blood vessels widen, allowing heat to escape through the skin. And you sweat, which cools you as it dries. If you need to heat up, the blood vessels narrow and the hairs stand up on end, trapping warm air around the body.

Below the dermis is fat, called the subcutaneous layer. Fat softens skin texture, cushions the skin, and protects it from cold temperatures.

Smart Skin Yet there's even more to this organ. It's a big player in the body's immune system. More than a physical barrier, skin contains protective white cells, and can "inform" the rest of the body if an incident occurs, so the rest of the body's white cells can be on alert. For information on our skin's relationship to the makeup we use, see Skin - What Goes On, Goes In at MyMakeupMirror.com

Amazingly, skin has a deep connection with our emotions. Call it "smart skin" if you will, it reflects our feelings and thoughts with blushing, hives, goosebumps, chills up the back, and "creeping" when something feels wrong. And often people will experience skin breakouts, rashes, or cold sores during times of stress.

Care Because it does absorb some external substances into the body, we need to be careful about the products we use. Synthetic fragrances and colors in body lotions and shampoos, for instance, can be irritants. Hair color can transfer easily through the scalp to the bloodstream. (You can find names of natural and vegan hair dyes at My Makeup Mirror

If you want to test your skin for sensitivity to a certain makeup or lotion, put a dab of the product on the inside of your wrist. If the skin gets irritated or red over the next 24 hours, you're better off avoiding the product.

As you know, sun is damaging. In addition to wearing a high SPF sunscreen, a wide-brimmed hat is recommended. If you're out in the summer between 10am and 2pm, you may also want to wear long sleeves and a top that covers as much as possible. Not very fashionable, perhaps, but definitely smart.