That we rarely succumb to infectious illness is downright miraculous, and our immune systems deserve most of the credit for that. The main function of this diffuse, interacting network of cells and chemicals is to combat pathogenic (disease-causing) microbes. It patrols the body for anything abnormal and potentially dangerous, such as cancer cells.
This complex and fascinating system affects your every waking hour, so learning how your immune system works will help you understand your body’s response to infection and shed light on how to boost your immunity. Let’s take a look inside. (See our Immune System chart for more details.)
Divide and Conquer
The immune system has two main divisions: the innate immune system, which needs no previous experience with intruders to dispatch them swiftly, and the acquired immune system, which requires contact and time in order to develop a specific response to a particular pathogen.
Innate. As its name suggests, the innate immune system is in place at birth. It reacts quickly and in a generalized fashion to any foreign invader. Physical barriers and reactions form a key component of this system: Skin and mucous membranes act like castle walls to keep out invaders. Eyelashes and nostril hairs trap dirt, microbes and pollen. Stomach acidity kills many microbes. Earwax keeps the inner ears healthy. Urination flushes out bacteria. Vomiting and diarrhea propel bad microbes from the intestinal tract. The gag, swallow and cough reflexes protect the body’s airways.
Fever is a second line of defense; it contributes by activating infection-fighting immune cells (“cytotoxic T cells”). White blood cells (called “leukocytes”) form another component of innate immunity. These include natural killer cells (which attack virus-infected cells and cancer cells); cells involved in allergic reactions; and several types of cells (“phagocytic cells”) capable of ingesting abnormal cells and bacteria, and other foreign material.
Some cells release “cytokines” — a large group of chemicals that facilitate communication among cells. Cytokines stimulate or inhibit activity of white blood cells, interfere with viral replication, and communicate with the brain. The brain, in turn, sends signals that influence the immune system and other bodily systems.
Here’s an example: Some cytokines contribute to the “inflammatory response,” the body’s nonspecific response to any irritant, including infection. The body’s characteristic redness, swelling, tenderness and warmth are evidence of increased blood flow and delivery of immunity goods to the site of the irritant. Meanwhile, cytokines also quickly reach the brain, prompting sleepiness to promote rest.
A steady but less dramatic rise in inflammatory chemicals over the course of the day leads to natural drowsiness at night. Chronic elevations of these chemicals — associated with chronic infections, autoimmune disorders, cardiovascular disease, diabetes, cancer, obesity, aging and T cell dysfunction — may disrupt learning and memory and lead to fatigue, anxiety and depression. Furthermore, chronic inflammation accelerates aging and contributes to a long list of chronic diseases.
Acquired. Compared with the prompt but nonspecific innate immune system, the acquired immune system develops reactions more slowly but creates a specific, enduring response to each type of intruder.
The acquired immune system contains white blood cells of its own called “lymphocytes.” The two main types of lymphocytes are T cells and B cells. T cells, which mature in the thymus, can be further divided into helper T cells, cytotoxic T cells, and regulatory T cells. After being activated by helper T cells, B cells mature and secrete antibodies (also called “immunoglobulins”), which bind to microbes and toxins and recruit other elements of the immune system to help eliminate viruses and bacteria from the body.
Connecting the Dots
It’s December. While hobnobbing at a holiday party, a co-worker exhales an influenza virus into your breathing space. A couple of days later, you’re coughing under the covers as your innate immune system struggles to contain the infection.
Meanwhile, B and T cells in your acquired system are learning to recognize this virus, mount an initial (albeit puny) response, and then create identical cells with immunologic memory against this particular viral strain. If you’re ever re-exposed, your immune system will respond so quickly and precisely that you won’t develop symptoms of that exact illness. That is why we usually get measles, mumps and similar diseases only one time. Flu viruses, on the other hand, are always evolving into new strains.
How Our Bodies Acquire Immunity
A body has two ways of developing immunity against a pathogenic microbe.
Active immunity. This immunity development entails exposure to the microbe. The microbe can be whole and alive, as usually happens when you’re exposed to someone who’s sick. Immunizations, on the other hand, involve exposure to only a small amount of a weakened or inactivated microbe, a fragment of the microbe, or an inactivated bacteria toxin (such as the tetanus toxin). Active immunity can last a lifetime when naturally acquired. Immunizations, however, often require booster shots to maintain effectiveness.
Passive immunity. This second immunity-acquiring process results from the receipt of antibodies from a person or animal previously exposed to the microbe. People exposed to rabies, hepatitis and other infectious diseases may be given injections of antibodies against the particular microbe. Additionally, a pregnant woman’s antibodies cross from her placenta to her fetus. After birth, her infant receives these immunologic compounds in breast milk. Passive immunity lasts for only a short period of time and does not result in immunologic memory — so, for example, a baby who gains immunity from her mother’s milk will not retain that immunity forever.
Our Microbial Companions
Some microorganisms threaten health while others support it. A host of beneficial microbes shapes an immune system and forms part of its nonspecific defense. About 90 percent of the cells in our bodies are inhabited by bacteria, fungi or other nonhuman cells. These microbes colonize “lining cells,” such as in the outer eye, gut, upper respiratory tract, lower urinary tract and vagina, to keep harmful bacteria at bay. Our intestines alone house up to 500 different bacterial species whose members outnumber our own cells by a factor of 10.
Among other benefits, these “good” microbes contribute to immune system defenses. They out-compete pathogenic bacteria in the same way that a well-planted garden leaves little room for weeds. They also amplify the immune system response by supporting the health of immune cells, antibodies and other compounds. One of the reasons antibiotics shouldn’t be dispensed without justification (such as to treat a serious bacterial infection) in humans and other animals is that they can also kill the resident good bacteria. That’s why diarrhea and an overgrowth of yeast in the vagina, mouth, intestinal tract and, for infants, diaper area are typical antibiotic side effects.
To restore beneficial microbes after taking antibiotics, try taking probiotic supplements, which are live microorganisms capable of colonizing the intestinal tract, and eating live-culture fermented foods, such as yogurt and kefir. Probiotic supplements reduce the odds of antibiotic-associated diarrhea by 60 percent relative to placebo. Live-culture fermented foods contain strains of probiotics, so can also help keep your bacterial balance in check. Start ingesting probiotics and probiotic-rich foods when you must take antibiotics, and continue for seven to 14 days.
Immune System Misfires
In order to attack pathogenic microbes and abnormal cells, the immune system has to distinguish between self and nonself. Sometimes this intricate system breaks down.
In autoimmunity, the immune system attacks the body’s own normal tissue. For instance, in Type 1 diabetes, the immune system attacks insulin-producing cells in the pancreas. In multiple sclerosis, the immune system attacks the myelin sheaths that insulate nerves in the brain and spinal cord.
In the case of hypersensitivity, the immune system is responding excessively or inappropriately. Everyday examples are hay fever and food or animal allergies. Food sensitivities (also called “food intolerances”), however, do not directly involve the immune system.
The immune system can also wear down. Immunodeficiency means the system is failing to fulfill its functions, increasing the body’s vulnerability to infection and cancer. Some immunodeficiency diseases are present at birth, while others are acquired from environmental exposures, chronic illness, malnutrition and certain medications. Viruses can impair immune function, a notorious example being the human immunodeficiency virus (HIV), the cause of acquired immune deficiency syndrome (AIDS).
Mood and Immunity
Several decades ago, scientists discovered that some of their allergic volunteers sneezed after viewing pictures of hay — without any exposure to its pollen. Then, in the mid-1970s, Robert Ader and Nicholas Cohen at the University of Rochester coined the term “psychoneuroimmunology” to describe the study of how nervous, hormone and immune systems interact. Their experiments showed that animals (human and nonhuman) could be conditioned to develop immune responses to innocuous agents, such as salt water.
We now know that depression and anxiety can aggravate allergies and asthma and dampen immune function. Infection and inflammation can also sour one’s mood. In contrast, positive emotions and laughter buoy immunity. A 2013 Japanese study examined the effect of a type of laughter therapy on people with advanced gastrointestinal cancer, who were scheduled for surgery and chemotherapy — treatments known to undermine already fragile immune status. Compared with no additional therapy, laughter therapy improved immune levels.
Chronic physical or psychological stress impairs immune function, but meditation offers a means to manage stress. A 2012 study from the University of Brasilia Laboratory of Cellular Immunology found that pranic meditation, which employs breathing and visualization techniques, enhanced the function of phagocytic cells. In a 2008 Loyola University of Chicago study, women with early-stage breast cancer who received mindfulness-based stress-reduction training had reduced levels of the stress hormone cortisol, and improved natural killer cell activity and cytokine levels, compared with women who didn’t receive the training.
Use Plant Power to Boost Your Immunity
The plant kingdom offers many allies for our embattled systems. Traditional healers have long treasured Asian and American ginseng. Studies indicate that both species positively modify immune function and help prevent some respiratory infections. Ginseng is threatened by over-harvesting, however, and many ginseng products on the market are from cultivated plants. To join the United Plant Savers in protecting wild American ginseng. You can also grow your own.
Other immune-tonic plants include Siberian ginseng, ginger, andrographis and medicinal mushrooms, such as shiitake, maitake and reishi. You can take your healing into your own hands by concocting your own teas and tinctures with these ingredients. (Read 19 Ways to Prevent and Treat Colds and Flu for more on how to boost your immunity with herbal and natural medicines.)
Now that you know the basics of how your immune system works, you can better understand how to sustain it. The days approaching fall and winter — cold and flu season — are a good time to think about nourishing your immune system. Eat whole foods, sleep amply, be social, stress less, exercise daily, and consider crafting some herbal immune boosters. Rest assured that your remarkable immune system is working day and night to protect you from infection. Now go out and enjoy this germy world.
Linda B. White, M.D., is the author of Health Now: An Integrative Approach to Personal Health and the co-author of 500 Time-Tested Home Remedies and the Science Behind Them and The Herbal Drugstore.