12 Fantastic Facts About the Immune System

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 and caused illnesses, like cancer, within the body. Here are 12 fascinating facts about your immune system.

1. It saves lives.
2. Before scientists understood the immune system, they blamed diseases on unbalanced humors.
3. The scientists who unraveled the immune system’s functions were bitter rivals.
4. White blood cells are the immune system’s greatest weapon.
5. The spleen helps your immune system work.
6. You have immune cells in all of your tissues.
7. Your immune might decide how friendly you feel.
8. You can “boost” your immune system by eating and sleeping well.
9. Your appendix might keep your gut in shape.
10. Gut bacteria has been shown to boost immune systems in mice.
11. Scientists are trying to harness the immune system‘s “Pac-Man” cells to treat cancer.
12. Immunotherapy is on the cutting edge of immune system research.

It 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. 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, told Mental Floss in 2019. “Yet, even though we're not aware of it, it's saving our lives every day, and doing a remarkably good job of it.”

Before scientists understood the immune system, they blamed diseases on unbalanced humors.

Before the germ theory of disease was discovered (more on that below), physicians diagnosed diseases according to the ratios of four humors of the body: melancholic, phlegmatic, choleric, or sanguine. These criteria, devised by the Greek philosopher Hippocrates, correlated to the four elements and linked to bodily fluids: Earth (black bile), air (blood), water (phlegm), and fire (yellow bile). They 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 probably did little to support the immune system's ability to resist infection.

The scientists who unraveled the immune system’s functions were bitter rivals.

Louis Pasteur and Robert Koch should have seen 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.

White 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.

The spleen helps your immune system work.

Though you can live without a spleen, an organ that lies between stomach and diaphragm, it's better to hang onto it for your immune function. According to Dr. Adriana Medina, a specialist in hematology and oncology in Washington, D.C., 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 said, “eventually they need to communicate with each other.” They do so in both the spleen and lymph nodes.

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 explained. 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 said.

Your immune might decide how friendly you feel.

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.

You can “boost” your immune system by eating and sleeping well.

You probably won’t realize that a pathogen has invaded your body until your immune system responds to it. While the white blood cells fend off the intruder, you may feel sick and have symptoms like a congested or runny nose, sore throat, fever, or body aches—but these unpleasant feelings are a sign that your immune system is working exactly as it should. “People think, ‘I’m so sick, this is terrible. Why doesn’t my immune system work?’ But with every one of these cold symptoms, that is your immune system at work,” Dr. Suzanne Cassel, an immunologist at Cedars-Sinai in Los Angeles, said.

A lot of over-the-counter products claim they help “boost” your immune system, but Cassel said most vitamins and supplements don't have any effect on the strength of your immune response. Habits that do help include eating a healthy, balanced diet, getting enough quality sleep, exercising regularly, and getting vaccinated against infectious diseases.

Your appendix might keep your gut in shape.

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 it back on track to recovery.

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.

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, a pediatric neurosurgeon now at the University of Utah Health, told Mental Floss in 2017.

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.

Immunotherapy is on the cutting edge of immune system research.

Recent 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 said 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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A version of this story was published in 2017; it has been updated for 2025.