Your immune system has likely been on your mind a lot lately. As the COVID-19 pandemic carries on, it continues to leave a puzzling array of outcomes in its wake: Some people die, others suffer severe complications, still others never show symptoms. This has left all of us struggling to understand the factors that contribute to this vast disparity.
One thing seems clear: A strong, balanced immune response can mitigate the effects of any infection, including the novel coronavirus. This means that bolstering your immune system may be a key to fending off illness when it strikes — or avoiding it altogether.
Here are key insights into how the human immune system operates, along with six things you can do to support your system’s most effective response to infectious agents.
The human immune system is divided into two primary parts: innate immunity and acquired or adaptive immunity.
“The main distinction between the two is that innate immunity needs no prior exposure to be activated,” explains integrative practitioner Robert Rountree, MD. The most elemental parts of the innate immune system are the body’s barriers — the walls of the skin and the gut, as well as the wet membranes of our eyes, ears, nose, gastrointestinal tract, and respiratory tract, which contain enzymes and free radicals that deter invaders.
“Like Teflon, our bodies slough off 99 percent of the things we’re exposed to,” he says.
If a pathogen does manage to breach one of these barriers, it encounters the next line of defense. Our bodies come preloaded with innate immune cells, including macrophages, neutrophils, dendritic cells, mast cells, and natural killer cells. These cells hang out at the body’s gateways — the skin, eyes, nose, throat, gut, bladder, respiratory tract, and so on — waiting for suspicious-looking strangers.
“The job of the innate immune system is surveillance — it’s the rapid responder,” says Rountree. Some of these immune cells, such as phagocytes and natural killer cells, take a good crack at eliminating the invader on their own through a range of mechanical and chemical actions.
Meanwhile, macrophages and other immune cells release inflammatory proteins called cytokines that lead to the typical signs of an inflammatory response, such as redness, heat, swelling, and pain. (For more on the double-edged sword of cytokines, see “When the Immune System Goes Rogue” at the end of this article.)
Neutrophils are responsible for releasing chemical signals to summon the heavy hitters of the adaptive immune system, such as the T cells and B cells. These mount a specific response to the unwelcome visitor like a sports team that has studied its opponent’s weak spots.
Once the adaptive immune system has been activated (sometimes days or weeks after the initial infection), it intensifies the inflammatory response started by the innate immune system and mobilizes an army of T cells and antibody-producing B cells to find and destroy the proliferating invaders.
T cells, which mature in the thymus gland, are divided into helper T cells, killer T cells, and suppressor T cells — each with its own important role.
B cells, which develop in bone marrow, circulate in the blood, hunting for ne’er-do-wells and producing hordes of antibodies specifically designed to inactivate the invader or mark it for destruction.
Vaccines take advantage of this antibody-producing action by introducing B cells to a weakened form of an enemy they may encounter later, giving them a chance to prepare neutralizing antibodies in advance.
Plenty of advice has been circulating on how to boost your immune system, but preventive-medicine specialist David Katz, MD, notes that a healthy immune system is not necessarily the most aggressive one. It’s the one that’s in balance.
“A healthy immune system is vigorous enough to defend the body against pathogens and rogue cells that produce cancer, but doesn’t overreact to mild provocations.”
Excess stress, poor sleep, exposure to toxins, excessive alcohol use, and a junk-food diet can all throw the immune system off kilter and contribute to chronic inflammation. This may increase your susceptibility to a host of illnesses, from upper-respiratory infections to cancer.
The good news is that many of the factors that support overall health also support a well-balanced immune system. Regular exercise; adequate sleep; stress reduction; a plant-heavy, whole-foods diet; and a healthy gut all help ensure your immune system is calm and ready for anything. Judicious use of supplements and herbs can lend extra support.
These are six of the most important tools for building and sustaining a robust and balanced immune response.
Regular physical activity is one of the keys to supporting immunity. David Nieman, DrPH, FACSM, an exercise immunologist at North Carolina’s Appalachian State University, explains that regular, moderate physical activity, such as brisk walking, jogging, cycling, or swimming for 30 to 60 minutes most days of the week, can help reduce the risk of chronic disease and respiratory infections.
“Most of our immune cells are in the peripheral lymphoid tissue like the spleen, thymus, lymph nodes, and bone marrow,” he says. “Those are like the Marines’ military base. When physical activity starts, important immune cells are recruited to start circulating in the blood looking for pathogens. Having these fighters [T cells, natural killer cells, and other immune cells] moving around outside the bases gives them an increased chance of finding and engaging the enemy.”
Within a few hours postexercise, the deployed immune cells return to base, waiting for the next bout of exercise to summon them again.
“When you exercise day after day, a summation effect occurs, and that increased immunosurveillance leads to a reduction in viral and bacterial load,” says Nieman. In a 2011 study, he found that people who were aerobically active on most days had a 43 percent reduction in days of illness with upper-respiratory infections compared with those who were inactive.
More of a good thing isn’t necessarily better, however. Nieman noticed when he was running a lot of marathons back in the 1980s that he and his fellow runners were more likely to get sick in the days following a race. In 1987 he followed 2,311 Los Angeles marathoners for two months before the race and a week after they had completed it. He found the runners were nearly six times more likely to contract an upper-respiratory infection in the week following the race than nonmarathoners.
“When you push beyond 90 minutes of unrelenting, high-intensity exercise, the glycogen stores in your muscles and liver get drained, stress hormones are released, and the immune system gets into an inflamed state,” he explains. “It’s like those Marines have been doing all that surveillance and not sleeping, so they can’t function as well.”
Specific nutritional and training strategies can mitigate stress to the immune system for elite athletes, soldiers, or others who need to train heavily, but for most of us, Nieman says, “we’re lucky that the most benefit comes from just 30 to 60 minutes of exercise most days of the week.”
Building muscle mass through strength training may also help support more robust immune function. Studies of mice suggest greater muscle mass helps support a healthy reserve of T cells, and human studies indicate that ample muscle mass correlates with better longevity. (For more on the value of strength training, see “Be Strong, Live Long”.)
Exercise has the added benefit of encouraging other immune-supporting behaviors on this list, such as stress reduction and sleep.
“Physical activity is the hub for wellness,” Nieman says. “If you’re active, you sleep better, control stress better, and tend to be more conscious of your diet. Make the decision to be active and many other things fall in line.”
While your conscious mind may experience sleep as a chance to rest, your body uses it to check off major items on its to-do list, including repairing and creating immune-system cells.
If you skimp on sleep, you may find yourself short on natural killer cells. One study found that a single night of sleep deprivation in 42 healthy male volunteers resulted in reduced immune response and T-cell cytokine production.
“Sleep affects our energy level and what we eat, and it’s directly linked to hormonal cycles,” notes Katz. “Sleep deprivation almost inevitably produces an imbalance in prevailing hormone levels, which in turn adversely affects the immune system.”
Studies have found that subjects who got a good night’s sleep following vaccination for hepatitis A produced twice as many antibodies as those who didn’t sleep, forming both a stronger initial adaptive-immune response and better long-term immunological memory. And a 2015 University of California, San Francisco study discovered that people who sleep six or fewer hours per night were four times more likely to catch a cold when exposed to the virus compared with those who slept seven or more hours.
Stress affects the immune system in a number of ways, from inhibiting sleep to increasing cortisol production. Because cortisol reduces the number of lymphocytes in the blood, chronic stress can suppress the immune system and leave you vulnerable to infection.
“Stress changes your body biochemically,” notes functional nutritionist Dee Harris, RDN. “It changes your whole immune system and the biochemistry of the gut and microbiome.”
Harris says she encourages her patients to use breath work and a spiritual practice of some kind to reduce stress. “Now is a great time to explore your spirituality and find your center,” she says, whether through a religious practice or meditation.
Harris also recommends taking a moment to feel gratitude before a meal. “That 15 to 30 seconds of relaxation before eating helps reset the vagal nerve and improves digestion.” (For more on stress relief, see “Reset Your Stress Response”.)
“Diet is all-important,” says Katz. “You’re building white blood cells, enzymes, and antibodies every day, and the food you eat is literally the source of your construction materials.”
A single meal can alter how immune cells respond to provocation, and the effects accumulate over hours, days, and weeks, he explains. “You can do a complete 180 and optimize a badly broken immune system in as little as weeks by improving your diet, so it’s a very immediate return on investment.”
Foods that dampen the immune system include highly processed or fried foods, those high in added sugar, and nonorganic foods grown with glyphosate, the chief ingredient in Roundup, a common herbicide that has been linked to cancer.
On the flip side, foods rich in polyphenols — beneficial plant compounds found in many vegetables, fruits, and legumes — support immune function. Rountree notes that the Mediterranean diet (plenty of colorful vegetables, nuts, and olive oil; moderate amounts of protein; and a little red wine with dinner) provides a good general template for immune-supportive eating.
Some immune system–balancing superstars to focus on:
Green tea is rich in polyphenols, including potent antioxidants called catechins that have antimicrobial properties and may help protect against influenza. It also contains quercetin, a flavonoid that Rountree calls a “time-honored immune-supportive agent.”
Berries are a potent source of immune-supporting flavonoids. “When you eat berries, most of these pigment molecules go to the colon, where bacteria break them down into smaller molecules that escape and circulate in the body, exerting antiviral effects,” says Nieman.
Turmeric gets its deep orange-yellow from curcumin, a compound that helps balance the immune system. It has a modulating effect on T cells, B cells, macrophages, and other immune cells, and can also enhance antibody response.
Garlic contains sulfuric compounds with a range of antimicrobial effects, such as inhibiting the biofilm formation of bacteria. It also has natural antiviral properties and can help reduce hypertension, one of the leading risk factors for COVID-19. (For more on garlic, see “Garlic”.)
Citrus fruits such as grapefruit, kiwi, and lemon deliver abundant vitamin C — one of the most important nutrients for the immune system, aiding in the formation of white blood cells. (For more on this essential nutrient, visit “What You Need to Know About Vitamin C”.)
Sauerkraut and other fermented foods contain lactic-acid bacteria, which produce compounds in the gut that spur the immune system into action. And cabbage itself is another excellent source of vitamin C.
Medicinal mushrooms are rich in beta-glucans, an immunomodulator that activates macrophages, natural killer cells, dendritic cells, and neutrophils. “Mushrooms like shiitake, oyster, and maitake have been shown to prime immune cells in published studies,” says Rountree. He recommends both eating shiitake mushrooms and taking a mushroom extract to support the immune system.
“Having healthy barriers [such as a healthy gut lining] is the No. 1 thing for a healthy immune system,” says Rountree. “And the healthier the microbiome, the better those membranes are going to be.”
Excessive use of alcohol and pain relievers like ibuprofen, as well as chemicals in our food, can all damage the integrity of the gut lining. This can lead to leaky gut and chronic inflammation.
Certain bacteria, such as Helicobacter pylori and Prevotella, can also cause imbalances in the microbiome, says Harris, in part by inhibiting the body’s ability to absorb critical minerals, including zinc, iron, and magnesium.
Harris always begins her work with a new patient by performing a GI panel that looks for parasites, candida, and any dysbiotic bacteria. She also checks for secretory immunoglobulin (or sIgA), an antibody that plays a critical role in immune defense against pathogens and can be depleted by antibiotics or an inflammatory diet.
To boost low sIgA levels, she recommends drinking gelatinous bone broth; eating mushrooms rich in beta-glucans, such as reishi, shiitake, and maitake; and taking Saccharomyces boulardii, a probiotic yeast.
Supplementing with a quality probiotic can also help bolster the immune system. Common probiotic strains, such as Lactobacillus and Bifidobacteria, have been shown to stimulate the activity of natural killer cells and enhance gut immunity. (For more on choosing the right probiotic, see “How Do I Find the Right Probiotic Supplement for Me?”.)
“Micronutrient deficiency is an epidemic,” says Harris, noting that the standard American diet, typically high in processed foods and low on fiber, causes nutrient depletion and stress on the body. A diet rich in vegetables, fruits, and whole grains will deliver many of the nutrients your body and immune system need, but certain supplements can offer extra support and help you reach optimal levels of vital nutrients.
Multivitamin: “For a healthy immune system, I think about the first part of the alphabet — vitamins A, B, C, D, and E,” says Rountree. “A quality multivitamin is the star.”
Harris recommends brands containing an active form of B12 and B9 (folate).
Vitamin D: Deficiency of vitamin D has been linked to both common and more severe upper-respiratory infections. While conventional medicine says vitamin-D blood levels over 30 ng/ML are adequate, Harris says that, from a functional-medicine perspective, optimal levels are much higher — between 60 and 80.
The sun is a generous provider of vitamin D: Those with fair skin may need as little as 10 to 15 minutes of direct sun exposure daily to make several thousand IUs, while those with darker skin tones may require up to two hours of exposure. Alternatively, most adults can supplement with up to 5,000 IUs of vitamin D3 daily, particularly in the dark winter months. (Work with your healthcare provider to check your D levels every few months.)
Zinc: “Zinc is important to the function of lymphocytes that defend against viruses,” says Katz. He recommends taking up to 30 mg of zinc daily, whether in a multivitamin or on its own. (See “Zinc Essentials” to learn more about how to optimize this vital mineral.)
NAC: Short for N-acetyl-L-cysteine, NAC helps the body make the antioxidant glutathione, which plays an important role in protecting against cellular damage and supporting immune health. In one clinical study, elderly people receiving 600 mg of NAC daily experienced fewer days of sickness from the flu than a placebo group, and showed fewer symptoms, despite similar infection rates.
Probiotic: The microbiome plays a critical role in immunity, says Katz. You can support a diverse and healthy microbiome by taking a quality probiotic. Harris recommends brands that contain a blend of Bifido and Lactobacillus bacteria and offer at least 100 billion CFUs.
Herbs: Herbs such as astragalus, echinacea, Chinese skullcap, and ashwagandha have been used as immuno-regulators in Ayurveda and Traditional Chinese Medicine for millennia. (For more on their protective and healing properties, see “9 Healing Herbs”.)