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Why Do Babies' Eyes Change Color?

In this episode of mental_floss' Big Questions, Craig answers a question from one of our YouTube fans: "Why do babies' eyes change color?"

Don't miss an episode—subscribe here! (Images provided by Shutterstock; transcript provided by Nerdfighteria Wiki.) 

Hi I'm Craig, my eyes might be brown, but my heart burns red for you, and this is mental_floss video. Today I'm gonna answer YouTuber doxysrkx's Big Question, "Why are babies born with blue eyes?" It's true that the majority of Caucasian newborns have blue eyes, but by adulthood only one in five have them. That's because eye color can change for white babies. African, Hispanic, and Asian babies are typically born with brown eyes which don't change color. Let's get started.

First, let's talk about what eye color is. Whether your eyes are brown, green, hazel, or blue has to do with something called melanin. It's basically a type of pigment which can also affect the colors of your skin and hair.

Your eyes contain melanocytes, which are cells that produce melanin. If there's a lot of melanin, the eyes are brown, if there's a medium amount, they're green or hazel, and a little bit means blue eyes. Another thing that affects eye color is the Tyndall effect, which is virtually identical to Rayleigh scattering, a concept you remember from our episode on whether blood is blue. I don't remember 'cause we do a million of these videos. And I'm usually drunk. But that explains why eyes may look different colors in different light; it doesn't affect whether eyes actually change color.

Newborns don't have the levels of melanin that they'll eventually have; the amount increases over time, which is why eyes often start blue then change to another color. They go from a small amount of melanin, which you'll remember causes eyes to be blue, to a larger amount which means they'll be a different color. Usually this happens around six months, but eyes can change color up to about three years old. And the reason this only happens for Caucasian babies is because they tend to be born with less pigment than other ethnicities.

So does this mean that your biology teacher was lying to you and eye color is more about melanin than genetics? Well no, it is genetic, they might've been lying to you about something else—I don't know, I don't know your biology teacher. But we now know that the genetics associated with eye color are more complicated than we once thought. You can't map it out on a simple chart because there are a handful of genetics that come together to affect eye color. In fact, experts predict that there are about 15 of them, and melanin production is just a little piece of the puzzle. 

Supposedly, there's a way to tell whether or not eye color will change by looking at a baby's eye from the side so that there's no light affecting your view. If there are hints of gold in the iris, you're rich! No, the eyes will probably become brown or green over time. If the eyes are still very blue, they'll probably stay that way. This needs to be properly studied though. And I'm not gonna do it. Someone will eventually.

Thanks for watching mental_floss video, which is made with the help of all of these cute widdle babies. If you have a big question of your own that you'd like answered, leave it below in the comments. I'll see you next week, with these brown eyes.

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The Body
12 Fantastic Facts About the Immune System
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The human body is an amazing thing. For each one of us, it's the most intimate object we know. And yet most of us don't know enough about it: its features, functions, quirks, and mysteries. Our series The Body explores human anatomy, part by part. Think of it as a mini digital encyclopedia with a dose of wow.

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, it also battles cells that have mutated due to illnesses, like cancer, within the body.

Here are 12 fighting 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 YOUR 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), the immune cytokine 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 ITS 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."

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Scientists May Have Found a Cure for Deadly White-Nose Syndrome in Bats
Ryan von Linden/New York Department of Environmental Conservation, Flickr // CC BY 2.0
Ryan von Linden/New York Department of Environmental Conservation, Flickr // CC BY 2.0

White-nose syndrome, a disease that affects insect-eating bats, is one of the most devastating wildlife diseases on record. But there may be a relatively simple way to stop it, according to new research: UV light.

As New Atlas reports, a new study from the U.S. Forest Service and the University of New Hampshire has found that only a few seconds of exposure to ultraviolet light causes permanent damage to the fungus that causes white-nose syndrome, Pseudogymnoascus destructans. The results were published in Nature Communications on January 2.

White-nose syndrome has killed millions of bats in the United States and Canada over the past decade, according to the USGS. Bats infected by the fungus use more energy during their winter hibernation than healthy bats, meaning they might run out of their energy reserves and die before spring comes. The infection causes dangerous physiological changes including severe wing damage, weight loss, and dehydration.

The P. destructans fungus can grow only in temperatures ranging from 39°F to 68°F, so it infects bats only when they're hibernating. But it's also hard to treat diseased bats as they hibernate, making it even more difficult for scientists to stop the disease. And stopping it is a big deal, not just for wildlife organizations but for governments and farmers, since the bats at risk are important predators that feed on crop-destroying insects. Previous research has shown that UV light can screen hibernating bats for white-nose syndrome—the skin lesions that form on the wings of infected bats glow orange-yellow under UV light—but this is the first study to show it can also be a treatment.

The researchers exposed six closely related Pseudogymnoascus species to UV light for a few seconds to see how the fungi would react. (P. destructans was the only pathogenic species involved.) They found that P. destructans lacked a key enzyme that helps it repair the DNA damage inflicted by exposure to UV light. Whereas other species weren't affected by the light, P. destructans exposed to a low dose of UV light had only a 15 percent survival rate. When that dosage was doubled (to what was still a moderate dose), the species had less than a 1 percent survival rate.

This extreme sensitivity to UV light could be a way for scientists to battle white-nose syndrome. But first they'll have to test the effects of the light on infected, hibernating bats, instead of just working with samples of the fungus in the lab. It's possible that the light could damage the bats' skin, killing off important species in their microbiome, or have some other unintended effect. But even as a preliminary finding, this is a hopeful step.

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