Imagine this: The love of your life is 10 inches shorter than you. This being a non-issue, the two of you get on with moving in together and starting a small brood of young humans of your own. Over time, something a little strange starts to occur. You seem to be shrinking just as your partner spurts up. When the dust settles, you maintain the height advantage but the distance between you is cut in half, down to just five inches.
This is analogous to what happens to your immune system when you co-parent. “You are completely changing the cells that constitute your immune system in a way as radical as changing your height,” says Adrian Liston, a researcher at the Translational Immunology Laboratory at VIB in Belgium. In 2016, Liston was part of the team that documented the physical composition of co-parents’ immune cells shifting to resemble their partners’ cells. Eventually, he says, co-parents end up with more in common immunologically than identical twins.
Are these changes for better or for worse? It’s a tough question to answer, because parenting brings both benefits and deficits. More critically, though, there is no such thing as an ideal immune system — their strength is in their diversity, and between healthy individuals it’s hard to say if one setup is better than another setup. Basically, it depends entirely on the context of what you need your immune system for, and what you need it to do.
It’s clear, however, that becoming a parent changes you fundamentally. Now we know that those changes take effect at the cellular level and define the structure of your inner defense systems. There’s still more we don’t know than we do about how this works, but here are five factors that likely affect it.
Your Behaviors and Habits
Startup founders would kill for the sort of disruptive power of a wailing newborn child. Humans tend to be creatures of habit, stuck in our daily routines. Our immune systems, under normal circumstances, tend to be fairly stable as well — perturbed by occasional assaults but with a tendency to return fairly quickly to baseline. A kid throws your life, and that of your co-parent, upside down, offering a unique opportunity to reshape behaviors as a family.
Liston expects that converging daily habits is the number one reason that co-parents’ immune systems start to look more similar after kids arrive. When couples move in together, they start to do more of the same things. One partner might quit smoking at the behest of the non-smoker. They’ll exercise together, or not at all. They’ll eat the same things, and be exposed to the same level of toxins inside the home. This is all the more true if you share a kid.
Taken together, these daily environmental impacts shape your immune system more significantly than anything else. Liston’s study only compares couples who live together and have young kids with random pairs, so it’s not possible to say at this point how much of the immune system changes are due to kids versus simply cohabiting.
However, it’s relatively safe to assume that roommates would converge slightly, but less than couples who live together, and much less than co-parents. Whether that’s good or bad thing depends largely on whether the lifestyle changes are, on balance, healthier or not.
The trillions of microbes that live on and inside your body interact with your immune system constantly. Consider them your immune system’s physical trainers, giving your immune cells a daily workout to keep them in shape. A growing body of evidence shows that microbial exposures are really important for the development of healthy immune systems, says Marie-Claire Arrieta, a pediatric microbiome researcher at the University of Calgary and co-author of Let Them Eat Dirt. Exposure to microbes in the birthing canal, in breastmilk, and from farm animals or pets have all been shown to protect children from illness later on by shaping their immune response.
The question of what kids do to their parents’ microbiomes hasn’t really been answered. Most of the research on adult microbiomes tends to show that perturbations are short-lived, with your unique colony settling back to baseline after some event, such as an acute illness. However, research has also shown that the microbial communities of families tend to converge, and more so if there’s a dog in the home. So it’s likely that kids, too, cause more bacteria to be shared between co-parents.
And that’s probably a good thing, but the evidence to say for sure doesn’t exist. Researchers in Arizona are working on it: they’re studying the impact of dogs on the health of the elderly, to see if sharing microbes with a canine protects against disease.
Your Exposure to Pathogens
When young children first make their way out in the world, they get exposed to pathogens their immune systems have never seen, and they tend to get sick. “As a parent of two young kids myself, I can say they expose me to a lot of their infections,” says Darragh Duffy, an immunologist with the Pasteur Institute in Paris. “That can be a good or a bad thing, it really depends on the context.”
Most of the things kids bring into your life are fairly minor, such as colds and flus. In theory, you should recover quickly and then have greater immune protection later in life, he says.
But if your kids are running you down to the point of being sick all the time, that could have severe consequences, says Liston. “You can get to the point where you’re getting chronic infections, and that has long-term impacts. If you are having a respiratory infection every two, three weeks, your lungs will be weaker, and that can have a very long-term impact.”
There’s also no evidence that these exposures, assuming you recover from them, strengthen your immune system overall. You’ll have some protection against the things you’ve seen before, but that doesn’t mean you’re better equipped to meet the challenge of a new threat later in life.
Your Sleep and Stress
Raising kids is tough, and if you’re not sleeping and you’re constantly stressed out, that’s not going to be great for your immune function. In terms of your immune system, most impacts can be good or bad depending on context, says Liston. But “stress is pretty unambiguously bad.”
It turns out that how healthy you are as a parent could actually depend on how you feel about being a parent.
Liston’s team tracked people in a Belgian village who got sick after sewage ended up in the drinking water, sampling their blood during and after. Those with low levels of stress markers in the blood tended to recover, but those that were highly stressed out fared much worse in the long run. “From a gastro infection that they should have been up and running two days later, some ended up developing irritable bowel syndrome, which gives you years and years of erratic cramps and diarrhea and constipation,” says Liston.
A 2015 study out of Northwestern University examined the impact of parental stress on immune function directly. The researchers found that parents with high levels of empathy suffered as their children displayed more depressive symptoms. Their immune systems became overactive, showing high levels of inflammation in response to a relatively benign threat. Chronic inflammation wears down your defenses and makes you more vulnerable to disease in the long run.
Erika Manczak, lead author of that study, who is now a postdoctoral fellow at Stanford University, says that while higher empathy appears to have benefits overall for both parents and children, parents should be careful not to sacrifice their own health for the sake of the kids.
“If you’re feeling exhausted and depleted, it’s actually really hard to be thoughtful and caring and supportive,” she says. “It’s really important to make sure that your needs are being met, because that actually makes you a better caregiver.”
Your Psychological Well-Being
It turns out that how healthy you are as a parent could actually depend on how you feel about being a parent. Rodlescia Sneed researches the link between social relationships, psychological health, and physical health at the College of Human Medicine at Michigan State University. As a graduate student at Carnegie Mellon University, she completed a study that found that parents were much less likely to get sick after exposure to a cold virus. People who already had antibodies in their system for that virus were excluded from the study, so it wasn’t a matter of having immunity from past infections.
While the study does not offer a direct answer for why this may be, Sneed expects that parenthood confers psychological benefits that protect against disease. Parents report greater meaning in life and greater life satisfaction as a result of having kids, and simply feeling good about your place in the world can keep you healthier. “It’s consistent with a larger body of evidence that does say that your social relationships are important for your health, and in particular your immune system,” Sneed says. The study found parents were less likely to get sick than non-parents, even after controlling for other possible factors, including marital status and size of social network.
There’s evidence to support this from the primate world, too. A 2016 study of rhesus macaques found that a monkey’s place in the social hierarchy impacted immune function. Those excluded from the group had a heightened inflammatory response, which is good for fighting bacterial infections, but can lead to health issues related to chronic inflammation down the line.
Most parents feel that having kids gives them a sense of purpose. This, in the end, might matter quite a bit more to your physical health than what germs your toddler is bringing home from daycare.