Is Your Immune System Smarter Than You Think?

Animation: Farah Hamade

Half-truths and oversimplifications have made the immune system seem either overzealous (hello, allergies and autoimmune diseases) or underprepared (stock up on disinfectant). Misinformation about vaccines has further clouded public understanding of its powers. We asked UCSF immunology experts to separate fact from fiction.

Myth #1 
The immune system’s only job is to attack.

Not even close. While it’s perhaps best known as the body’s defense system, the human immune system also contributes to tissue development and repair, metabolism, and wound healing.

In fact, the immune system performs as many as a dozen distinct health-promoting functions, says Max Krummel, PhD, the Robert E. Smith Professor of Experimental Pathology at UCSF.

For example, scientists used to think macrophages (the so-called “big eaters” of the immune system, known for killing microbes and removing dead cells) just gobbled entire cells, Krummel says. Turns out, they can actually be quite gentle, pinching off bits of nearby healthy cells, to gather intel on your health status. “The immune system has this role in knowing who you are,” Krummel says.

If scientists can somehow train macrophages to identify tumors, this discovery could lead to a new way to fight cancer. Tumors often evade the immune system for months or years, avoiding detection by growing “slowly and cautiously,” Krummel says. “This is the trick that cancer is using,” he says. “We’re figuring out how to dial it back.”

Myth #2 
If you have a healthy immune system, you don’t need vaccines.

The opposite is true. You need a healthy immune system to respond well to vaccines, says Mark Ansel, PhD ’01, a UCSF professor of microbiology and immunology.

Vaccines contain disarmed pieces of disease-causing microbes that spur the body to stage a defense without causing illness, Ansel says. After vaccination, a healthy immune system assembles antibodies to protect you from future infection. Your body retains that immune memory so it can fight back later, sometimes without you even realizing you were infected.

Unfortunately, people with severely compromised immune systems, such as newborns, transplant patients, or those on chemotherapy, can’t mount a robust vaccine response. That means they won’t be protected from severe illness and will be more likely to catch and spread the infection if they encounter the germs later. This is where community immunity becomes critical. “You need a large enough number of people vaccinated, so the disease doesn’t spread,” Ansel says. In that case, he adds, “if a few people are immunosuppressed, they probably won’t get infected because it won’t be going around.”

But what about the argument that healthy bodies are meant to fight off infections naturally? While that’s true to an extent, Ansel says, it’s not always the case. Even otherwise healthy people have died from vaccine-preventable diseases. And sometimes the consequences of being infected last far beyond the initial illness, such as in long COVID.

“If you look across the spectrum of infectious diseases, there are lots of cases where you can survive an infection, but it’s terrible,” Ansel says. “It’s a much safer world since we’ve had vaccines.”

Myth #3 
Too many vaccines overwhelm the immune system.

Nope. There is no evidence to suggest this, Ansel says.

If you’ve ever experienced the misery of a co-infection — getting the flu and pneumonia at the same time, for instance — then you know the immune system can fight off two illnesses at once. The same is true for vaccine responses, Ansel says. The antibodies we generate to fight the flu and pneumonia, or their vaccines, function independently and can work in tandem without damaging your immune system.

“Our immune systems are well equipped to handle multiple infections,” Ansel says. “For the same reason, they’re well equipped to handle multiple vaccines.”

The actual risks of vaccines are much more specific, Ansel says, and they rarely outweigh the benefits. Some vaccines can induce allergic responses in certain people. Perhaps the best-known example is the flu vaccine, some versions of which are made with eggs. People with egg allergies should avoid these versions and opt for alternate flu shots.

In extremely rare cases, the body’s immune response to a vaccine can adversely affect other organs, Ansel says. For instance, a small number of teenage boys experienced myocarditis, an inflammation of the heart muscle that can be related to an immune response, after receiving the COVID vaccine booster. It turns out that unvaccinated exposure to the COVID virus itself can also cause myocarditis.

“The benefit to individuals, and certainly to society, of using vaccines vastly outweighs the risks,” Ansel says. “But these are medications, and some people have bad reactions. We have to use them safely.”

Myth #4 
If we want kids to have fewer allergies, we should expose them to more allergens.

Kind of! It’s true that early exposure to certain allergens can help prevent allergies — essentially an inappropriate immune response — from developing later on. Peanuts are a prime example. A decade ago, researchers from the Immune Tolerance Network at UCSF debunked the long-held belief that avoiding peanuts in infancy prevented peanut allergies.

But that doesn’t mean we should overcorrect, says Tiffany Scharschmidt, MD, a professor and vice chair of research in the Department of Dermatology at UCSF. “I have a picture of my son when he was 10 months old about to put a handful of dirt from our yard into his mouth,” she says. “I snapped it, then said, ‘Let’s not eat that.’ But I’m totally fine with him playing in it.” That’s because a little exposure to potential allergens early in life is actually a great way to teach the immune system what’s safe.

A child’s immune system is meant to interpret the world around it. In fact, early-life immune cells are more likely to respond to potential toxins and bacteria with tolerance, almost curiosity, than with inflammation. “If you shelter [a child] from these antigens,” Scharschmidt says, “they miss that essential learning opportunity.” (While this is true for most children, she adds, some with a heightened allergic predisposition will respond with inflammation, so it’s best to consult with your pediatrician.)

There’s also evidence that having a dog in childhood can help prevent asthma and allergies. Ultimately, that wasn’t enough to overcome the practicalities of dog ownership for Scharschmidt and her family. Still, she lets her brother’s and neighbors’ dogs lick and play with her children. “It’s another immune education opportunity,” she says. “It’s a balance.”