13 Facts About Skin

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Skin isn't just the outermost layer of our bodies. Without it, we couldn't do most of the things we take for granted, like breathing, moving, and keeping the body's inner workings where they belong. And while skin also evolved to keep pathogens and other bad stuff out of our bodies, consumers spend millions of dollars on products to penetrate that defense (with mixed results). Read on for more fascinating facts about the skin.

1. YOUR SKIN HAS THREE DISTINCT LAYERS.

Skin is considered an organ in its own right. It's comprised of three layers: the waterproof top layer, the epidermis; a middle layer of tougher connective tissue, hair follicles, and glands called the dermis; and the inner layer, the hypodermis, which is mostly fat and connective tissue that supports the skin's structure and attaches it to muscles.

2. SKIN COLOR IS DETERMINED BY CELLS IN THE EPIDERMIS.

Those cells are known as melanocytes, which secrete a pigmented substance called melanin; the more melanin in the cells, the darker the skin. Having too little or too much melanin can lead to some skin color disorders: On one end of the spectrum are conditions like vitiligo—which occurs when some melanocytes lose the ability to produce melanin, resulting in whitish patches on the skin—and albinism, a condition in which melanocytes don't produce any melanin. On the other end is hyperpigmentation—the presence of excess melanin, which can cause darker patches of skin.

3. YOUR SKIN COULD WEIGH MORE THAN 20 POUNDS.

"Your skin accounts for 15 percent of your body weight," says Toral Patel, M.D., a board-certified dermatologist and supervising physician at D&A Dermatology in Chicago and a clinical instructor of medicine at Northwestern University. This makes it your body's largest organ.

According to that calculation and data from the Centers for Disease Control and Prevention, an average American woman weighs 168.5 pounds and carries more than 25 pounds of skin. An average man weighing 195.7 pounds will have nearly 30 pounds of skin.

4. YOUR SKIN RENEWS ITSELF EVERY 28 TO 30 DAYS.

New cells are created in that deep layer of the skin and take about four weeks to rise to the surface. There, they grow hard and then shed. This process, in which old skin is sloughed off and replaced by newer skin, might occur more than 1000 times over the average American's lifespan. But all skin is not created equal: Its thickness varies naturally among all areas of the body. Thickness can also be affected by age, gender, and habits (like smoking) that can change the cells' elasticity and other traits. According to Patel, the skin on the soles of your feet is up to seven times thicker than the skin of your eyelids.

5. TATTOOS STAY PUT, THANKS TO CELLS CALLED MACROPHAGES.

If your skin cells shed every month, how do tattoos stick around? It turns out to be a function of your immune system. The puncture of the tattoo needle causes inflammation in the dermis, the skin's middle layer. In response, white blood cells known as macrophages are sent in to help heal the damage. These macrophages "eat" the dye and can pass it to newer macrophages when they die off, so the pigment is essentially transferred from one cell to another. Any leftover pigment is soaked up by fibroblasts, which are longer-lasting skin cells that don't regenerate as often. Only lasers designed for tattoo removal are strong enough to kill off the macrophages and fibroblasts that hold the dye.

6. YOUR SKIN IS HOST TO BILLIONS OF CREATURES.

Your skin hosts a microbiome that can contain more than 1000 types of bacteria (along with other microbes, viruses, and pathogens). These "tiny ecosystems," as Patel describes them, are mostly friendly bacteria that work in concert with our bodies for many beneficial purposes, including wound healing, reducing skin inflammation, and assisting the immune system to help fight infection. These bacteria were once thought to outnumber your own cells 10 to one, but more recent research has found the ratio is closer to 1:1.

7. ANCIENT EGYPTIANS PUT SALT (AND OTHER FOODS) IN THEIR WOUNDS.

Injuring or breaking the skin's dermis, the layer below the epidermis, can expose the inner tissues to pathogens. To prevent infections from reaching any further into the skin, body fat, or muscle, ancient Egyptians cared for topical wounds with salt (yes, really!), fresh meat, moldy bread, and onions.

While these may seem like unsanitary things to put on a cut, modern research has found that there was actually merit in their methods. With its high iron content, meat was a good blood coagulant and recommended for the first day of a wound, according to a 2016 paper in the Journal of the German Society of Dermatology. Salt and onions are both astringent, which can stop blood flow. Moldy bread likely had antibacterial properties—a very early form of penicillin, you might say. Skin wounds would then be sealed with a combination of oils, fats, honey, and plant fibers.

8. YOUR BODY'S FLUID BALANCE DEPENDS ON SKIN.

Your skin is a significant shield against billions of tiny microbes and pathogens. But just as importantly, skin keeps fluids in. Another way to think of this, Patel says, is that your skin resembles a brick and mortar pattern. The bricks are the cells. The mortar is made up of lipids, fatty acids, and other sticky proteins that form the watertight layer. "If you have any ‘holes' in skin where moisture can escape, which are more susceptible to damage, that leads to dryness, cracking, and inflammation," Patel says.

People who have suffered burns often have fluid-balance problems, says Robert T. Brodell, M.D., professor of dermatology at University of Mississippi Medical Center in Jackson, Mississippi. "Fluids are seeping out, and they can't keep them balanced internally," he tells Mental Floss. This can be incredibly dangerous, because fluid loss can cause the heart to stop pumping blood to the rest of the body. Dehydration, hypertension, and other problems may also occur when skin is injured.

9. A SKIN CONDITION CAN PUT YOU AT GREATER RISK OF OTHER DISEASES.

Psoriasis is an autoimmune condition in which the skin cells in an affected area grow rapidly, leading to excess skin buildup, inflammation, and a red and scaly rash. While it can be uncomfortable to live with the condition on its own, studies [PDF] have shown that inflammation of the skin can lead to inflammation of other tissues and internal organs, and eventually certain diseases. For example, psoriasis has been linked to a greater risk for heart disease, as well as diabetes, Crohn's disease, metabolic syndrome, and other conditions thought to be correlated with inflammation.

Patel says that association makes treatment even more important: "If one organ is inflamed, you have to make sure another isn't."

10. YOUR LEGS MAY BE THE DRIEST PART OF YOUR BODY.

Unless you live in the tropics, you've probably noticed that the skin of your lower legs becomes drier in winter—and there's a biological reason for that. "You have fewer oil glands on your legs than any other area of your body," Brodell tells Mental Floss. Oil (or sebaceous) glands, found near the dermis's border with the epidermis, secrete an oily substance called sebum that lubricates skin and hair. As people age, the glands secrete less oil, and that means drier skin. Winter's low humidity and our tendency to spend more time around heat sources dries out skin even more.

The solution is to install a humidifier or apply some moisturizer. Certain skincare products, such as those with emulsifiers like sodium laureth sulfate, can also dry out or irritate your skin, so read your labels carefully.

11. OVERHEATING IS A RISK IF YOU LACK SWEAT GLANDS.

Both types of sweat glands are also located in the dermis. Eccrine glands, found all over the body, emit sweat directly through pores in the epidermis. Apocrine glands release sweat along hair follicles, so it's no surprise that these glands are concentrated in the hairiest parts of the body—head, armpit, and groin. Both types help regulate body temperature: In hot conditions, the glands release water and fatty liquids to cool the skin.

A lack of sweat glands puts people in danger of overheating. Those with a condition known as anhidrotic ectodermal dysplasia have few to no sweat glands, so they can't properly cool off when the body overheats. "They get heatstroke easily," Brodell says. A subset of people with this disorder suffer from immunodeficiency. They produce low levels of antibodies and infection-fighting immune T- and B-cells, so they are more prone to skin and lung infections.

12. YOUR GUT AND YOUR SKIN ARE SYMBIOTIC.

The gut and the skin never come into direct contact with one another, yet research shows that the gut has a profound impact on the skin.

"The skin becomes very unhealthy when the microbiome of the gut goes into a state of dysbiosis," meaning when something attacks the gut's good bacteria, says Gregory Maguire, Ph.D., a former professor of neuroscience at UC San Diego and the founder and chief scientific officer of BioRegenerative Sciences, a stem-cell technology company.

Dysbiosis can lead to inflammation, irritation, rashes, and pain. "There's good evidence that eczema [or] atopic dermatitis is partially due to dysbiosis of the gut and skin," he says.

In a 2017 paper published in the Archives of Dermatological Research, Maguire writes that normal gut bacteria can actually calm the body's response to stress. A reduction in the release of the stress hormone cortisol, which is thought to cause breakouts, also reduces the chance of skin irritation—all thanks to microbes in your intestine.

13. RESEARCHERS ARE USING "GOOD" BACTERIA TO TREAT ACNE.

When the skin's pores get clogged with sebum from the sebaceous glands and dead cells, a condition usually associated with hormonal changes, you've got acne. Clogged pores that stay closed are called whiteheads; if the pore opens and reveals the gunk inside, it's a blackhead. (The medical term for a blackhead, an "open comedo," stems from a Latin phrase alluding to "worms which devour the body." But don't worry, blackheads are not actual worms living in your face.)

While acne may seem like a rite of passage associated with puberty, researchers are experimenting with fighting "bad" bacteria (in this case, Propiobacterium acnes, which is linked to acne breakouts) with "good" bacteria, also known as probiotics. "One of the things [probiotics] do is ferment things on the skin like ammonia and nitrites, and metabolize it and turn it into other chemicals that are beneficial to the stem cells in your skin," Maguire explains. A 2015 study in the Journal of Women's Dermatology and other research has found that applying topical probiotics like Streptococcus salivarius and Streptococcus thermophiles inhibits P. acnes and may make skin more resilient against it in the long run.

12 Fantastic Facts About the Immune System

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monkeybusinessimages/iStock via Getty Images

If it weren't for our immune system, none of us would live very long. Not only does the immune system protect us from external pathogens like viruses, bacteria, and parasites, but it also battles cells that have mutated due to illnesses, like cancer, within the body. Here are 12 fascinating facts about the immune system.

1. The immune system saves lives.

The immune system is a complex network of tissues and organs that spreads throughout the entire body. In a nutshell, it works like this: A series of "sensors" within the system detects an intruding pathogen, like bacteria or a virus. Then the sensors signal other parts of the system to kill the pathogen and eliminate the infection.

"The immune system is being bombarded by all sorts of microbes all the time," Russell Vance, professor of immunology at University of California, Berkeley and an investigator for the Howard Hughes Medical Institute, tells Mental Floss. "Yet, even though we're not aware of it, it's saving our lives every day, and doing a remarkably good job of it."

2. Before scientists understood the immune system, illness was chalked up to unbalanced humors.

Long before physicians realized how invisible pathogens interacted with the body's system for fighting them off, doctors diagnosed all ills of the body and the mind according to the balance of "four humors": melancholic, phlegmatic, choleric, or sanguine. These criteria, devised by the Greek philosopher Hippocrates, were divided between the four elements, which were linked to bodily fluids (a.k.a. humors): earth (black bile), air (blood), water (phlegm) and fire (yellow bile), which also carried properties of cold, hot, moist, or dry. Through a combination of guesswork and observation, physicians would diagnose patients' humors and prescribe treatment that most likely did little to support the immune system's ability to resist infection.

3. Two men who unraveled the immune system's functions were bitter rivals.

Two scientists who discovered key functions of the immune system, Louis Pasteur and Robert Koch, should have been able to see their work as complementary, but they wound up rivals. Pasteur, a French microbiologist, was famous for his experiments demonstrating the mechanism of vaccines using weakened versions of the microbes. Koch, a German physician, established four essential conditions under which pathogenic bacteria can infect hosts, and used them to identify the Mycobacterium tuberculosis bacterium that causes tuberculosis. Though both helped establish the germ theory of disease—one of the foundations of modern medicine today—Pasteur and Koch's feud may have been aggravated by nationalism, a language barrier, criticisms of each other's work, and possibly a hint of jealousy.

4. Specialized blood cells are the immune system's greatest weapon.

The most powerful weapons in your immune system's arsenal are white blood cells, divided into two main types: lymphocytes, which create antigens for specific pathogens and kill them or escort them out of the body; and phagocytes, which ingest harmful bacteria. White blood cells not only attack foreign pathogens, but recognize these interlopers the next time they meet them and respond more quickly. Many of these immune cells are produced in your bone marrow but also in the spleen, lymph nodes, and thymus, and are stored in some of these tissues and other areas of the body. In the lymph nodes, which are located throughout your body but most noticeably in your armpits, throat, and groin, lymphatic fluid containing white blood cells flows through vein-like tubules to escort foreign invaders out.

5. The spleen helps your immune system work.

Though you can live without the spleen, an organ that lies between stomach and diaphragm, it's better to hang onto it for your immune function. According to Adriana Medina, a doctor who specializes in hematology and oncology at the Alvin and Lois Lapidus Cancer Institute at Sinai Hospital in Baltimore, your spleen is "one big lymph node" that makes new white blood cells and cleans out old blood cells from the body.

It's also a place where immune cells congregate. "Because the immune cells are spread out through the body," Vance says, "eventually they need to communicate with each other." They do so in both the spleen and lymph nodes.

6. You have immune cells in all of your tissues.

While immune cells may congregate more in lymph nodes than elsewhere, "every tissue in your body has immune cells stationed in it or circulating through it, constantly roving for signs of attack," Vance explains. These cells also circulate through the blood. The reason for their widespread presence is that there are thousands of different pathogens that might infect us, from bacteria to viruses to parasites. "To eliminate each of those different kinds of threats requires specialized detectors," he says.

7. How friendly you're feeling could be linked to your immune system.

From an evolutionary perspective, humans' high sociability may have less to do with our bigger brains, and more to do with our immune system's exposure to a greater number of bacteria and other pathogens.

Researchers at the University of Virginia School of Medicine have theorized that interferon gamma (IG), a substance that helps the immune system fight invaders, was linked to social behavior, which is one of the ways we become exposed to pathogens.

In mice, they found IG acted as a kind of brake to the brain's prefrontal cortex, essentially stopping aberrant hyperactivity that can cause negative changes in social behavior. When they blocked the IG molecule, the mice's prefrontal cortexes became hyperactive, resulting in less sociability. When they restored the function, the mice's brains returned to normal, as did their social behavior.

8. Your immune system might recruit unlikely organs, like the appendix, into service.

The appendix gets a bad rap as a vestigial organ that does nothing but occasionally go septic and create a need for immediate surgery. But the appendix may help keep your gut in good shape. According to Gabrielle Belz, professor of molecular immunology at the Walter and Eliza Hall Institute of Medical Research in Melbourne, Australia, research by Duke University's Randal Bollinger and Bill Parker suggests the appendix houses symbiotic bacteria that are important for overall gut health—especially after infections wipe out the gut's good microbes. Special immune cells known as innate lymphoid cells (ILCs) in the appendix may help to repopulate the gut with healthy bacteria and put the gut back on track to recovery.

9. Gut bacteria has been shown to boost immune systems in mice.

Researchers at the University of Chicago noticed that one group of mice in their lab had a stronger response to a cancer treatment than other mice. They eventually traced the reason to a strain of bacteria—Bifidobacterium—in the mice's guts that boosted the animals' immune system to such a degree they could compare it to anti-cancer drugs called checkpoint inhibitors, which keep the immune system from overreacting.

To test their theory, they transferred fecal matter from the robust mice to the stomachs of less immune-strengthened mice, with positive results: The treated mice mounted stronger immune responses and tumor growth slowed. When they compared the bacterial transfer effects with the effects of a checkpoint inhibitor drug, they found that the bacteria treatment was just as effective. The researchers believe that, with further study, the same effect could be seen in human cancer patients.

10. Scientists are trying to harness the immune system's "Pac-Man" cells to treat cancer.

Aggressive pediatric tumors are difficult to treat due to the toxicity of chemotherapy, but some researchers are hoping to develop effective treatments without the harmful side effects. Stanford researchers designed a study around a recently discovered molecule known as CD47, a protein expressed on the surface of all cells, and how it interacts with macrophages, white blood cells that kill abnormal cells. "Think of the macrophages as the Pac-Man of the immune system," Samuel Cheshier, lead study author and assistant professor of neurosurgery at Stanford Medicine, tells Mental Floss.

CD47 sends the immune system's macrophages a "don't eat me" signal. Cancer cells fool the immune system into not destroying them by secreting high amounts of CD47. When Cheshier and his team blocked the CD47 signals on cancer cells, the macrophages could identify the cancer cells and eat them, without toxic side effects to healthy cells. The treatment successfully shrank all five of the common pediatric tumors, without the nasty side effects of chemotherapy.

11. A new therapy for type 1 diabetes tricks the immune system.

In those with type 1 diabetes, the body attacks its own pancreatic cells, interrupting its normal ability to produce insulin in response to glucose. In a 2016 paper, researchers at MIT, in collaboration with Boston's Children's Hospital, successfully designed a new material that allows them to encapsulate and transplant healthy pancreatic "islet" cells into diabetic mice without triggering an immune response. Made from seaweed, the substance is benign enough that the body doesn't react to it, and porous enough to allow the islet cells to be placed in the abdomen of mice, where they restore the pancreatic function. Senior author Daniel Anderson, an associate professor at MIT, said in a statement that this approach "has the potential to provide [human] diabetics with a new pancreas that is protected from the immune system, which would allow them to control their blood sugar without taking drugs. That's the dream."

12. Immunotherapy is on the cutting edge of immune system research.

Over the last few years, research in the field of immunology has focused on developing cancer treatments using immunotherapy. This method engineers the patient's own normal cells to attack the cancer cells. Vance says the technique could be used for many more conditions. "I feel like that could be just the tip of the iceberg," he says. "If we can understand better what the cancer and immunotherapy is showing, maybe we can go in there and manipulate the immune responses and get good outcomes for other diseases, too."

Why Do People Get Ice Cream Headaches?

CharlieAJA/iStock via Getty Images
CharlieAJA/iStock via Getty Images

Reader Susann writes in to ask, "What exactly is the cause of brain freeze?"

You may know an ice cream headache by one of its other names: brain freeze, a cold-stimulus headache, or sphenopalatine ganglioneuralgia ("nerve pain of the sphenopalatine ganglion"). But no matter what you call it, it hurts like hell.

Brain freeze is brought on by the speedy consumption of cold beverages or food. According to Dr. Joseph Hulihan—a principal at Paradigm Neuroscience and former associate professor in the Department of Neurology at the Temple University Health Sciences Center, ice cream is a very common cause of head pain, with about one third of a randomly selected population succumbing to ice cream headaches.

What Causes That Pain?

As far back as the late 1960s, researchers pinned the blame on the same vascular mechanisms—rapid constriction and dilation of blood vessels—that were responsible for the aura and pulsatile pain phases of migraine headaches. When something cold like ice cream touches the roof of your mouth, there is a rapid cooling of the blood vessels there, causing them to constrict. When the blood vessels warm up again, they experience rebound dilation. The dilation is sensed by pain receptors and pain signals are sent to the brain via the trigeminal nerve. This nerve (also called the fifth cranial nerve, the fifth nerve, or just V) is responsible for sensation in the face, so when the pain signals are received, the brain often interprets them as coming from the forehead and we perceive a headache.

With brain freeze, we're perceiving pain in an area of the body that's at a distance from the site of the actual injury or reception of painful stimulus. This is a quirk of the body known as referred pain, and it's the reason people often feel pain in their neck, shoulders, and/or back instead of their chest during a heart attack.

To prevent brain freeze, try the following:

• Slow down. Eating or drinking cold food slowly allows one's mouth to get used to the temperature.

• Hold cold food or drink in the front part of your mouth and allow it to warm up before swallowing.

• Head north. Brain freeze requires a warm ambient temperature to occur, so it's almost impossible for it to happen if you're already cold.

This story has been updated for 2019.

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