Allergies and hormesis

Do you have allergies? Are you sensitive to certain foods or chemicals?  If so, you are part of an epidemic explosion in the incidence of allergies and sensitivities in the U.S. and Western societies. The allergy epidemic is frequently blamed on the profusion of pollutants and toxic man-made chemicals in modern industrial society.  And the conventional medical approach to dealing with allergies is to avoid exposure to allergens, and to block allergic reactions by using antihistamines.

But there is an alternative explanation and a more effective treatment, consistent with the theory of hormesis.  The explanation is called the hygiene hypothesis and the treatment is called allergen immunotherapy.  I’ll discuss these both shortly, but first let’s look at what is really behind the outbreak of allergies in the modern world.

The allergy explosion. In his book “Why Things Bite Back: Technology and the Revenge of Unintended Consequences“, Edward Tenner noted that hay fever was virtually unknown in the year 1800.  But by the end of the nineteenth century, hay fever and other allergies had become widespread.  Yet, according to Tenner, this did not seem to be a consequence of industrialization or urbanization per se:

In fact, as hay fever and other allergies multiplied in the nineteenth century, it was not working class children growing up amid industrial haze but instead the scions of the best households that were affected. Epidemiologists are beginning to believe that large families, messy play, and early infections could have helped condition children’s immune systems not to gear up against a common substance like pollen when they first encountered it.  The protein that mediates hay fever, IgE, appears designed to defend the body against worm infestation. The allergist and historian Michael Emanuel has speculated that hay fever results from IgE deprived of its original target, noting that “man evolved with his parasites and their may be a price to pay for their removal.” (WTBB, pp. 102-3).

More recently, over the past several decades in the U.S., the prevalence of atopic dermatitis  in children has skyrocketed. A high percentage of these kids go on to develop full-blown allergies and asthma.  And paradoxically, wider use of medication to control allergies does not seem to be helping.  According to a 1996 article in Environmental Health Perspectives, citing Peter Gergen of the National Institite of Allergy and Infectious Disease, we are confronted with a disturbing reality:

Trends in Asthma Prevalence, 1982-1993

[D]uring the last three decades, asthma prevalence and morbidity in the United States has been rising. “The paradox of asthma is that we’ve had good treatment and quite adequate medications, and yet we’re still having this problem,” said Gergen… The increase in asthma is not unique to the United States. Asthma appears to be growing worse in other economically developed countries as well.

But the article follows the conventional way of thinking, going on to put the blame on environmental factors prevalent in urban areas:

Spurred by the alarming statistics, researchers are focusing on direct exposures to allergens indoors where people are spending more of their time. Allergen levels are thought to be higher in less well-ventilated homes, where moisture accumulates, allowing mildew and molds to grow. Research shows that cumulative exposure to dust mites, which live in bedding, upholstery, and carpets, causes some people to develop allergic sensitivity, including asthma and airway hyperresponsiveness. The levels of cockroach antigen generally found in suburban homes are too low to sensitize individuals, but the 10-fold higher levels found in inner-city dwellings are enough to cause sensitization and appear to be associated with asthma. “We’re also concerned about second-hand tobacco smoke,” said Alfred Munzer, pulmonary specialist at Washington Adventist Hospital and former president of the ALA. “There is increasing evidence that childhood exposure to environmental smoke can be a predisposing factor to developing asthma.”

Yet, in contradiction to the above statement about the effects of cigarette smoke, the incidence in allergies has been rising at the same time as smoking is on the decline. Furthermore, exposure to environmental pollutants and indoor allergens like dust mites cannot explain the increase in food allergies.  In fact the rise of food allergies might give us a partial clue:  According to a recent CNN article, there is evidence that the standard Western diet has altered intestinal flora and weakened our immune systems:

A study in Proceedings of the National Academy of Sciences compared the gut bacteria from 15 children in Florence, Italy, with gut bacteria in 14 children in a rural African village in Burkina Faso. They found that the variety of flora in these two groups was substantially different. The children in the African village live in a community that produces its own food. The study authors say this is closer to how humans ate 10,000 years ago. Their diet is mostly vegetarian. By contrast, the local diet of European children contains more sugar, animal fat and calorie-dense foods. The study authors posit that these factors result in less biodiversity in the organisms found inside the gut of European children. The decrease in richness of gut bacteria in Westerners may have something to do with the rise in allergies in industrialized countries, said Dr. Paolo Lionetti of the department of pediatrics at Meyer Children Hospital at the University of Florence. Sanitation measures and vaccines in the West may have controlled infectious disease, but the decreased exposure to a variety of bacteria may have opened the door to these other ailments.

Even establishment allergists like Dr. Hugh Sampson and Dr. Scott Sicherer of Mt. Sinai Medical Center in New York are unable to explain the dramatic rise in food allergies, and are beginning to doubt the advice given to parents to avoid exposing their children to milk, eggs, peanuts, tree nuts, or seafood at a young age.  According to a recent article in the New Yorker:

For most of his career, he believed, like most allergists, that children are far less likely to become allergic to problematic foods if they are not exposed to them as infants. But now Sampson and other specialists believe that early exposure may actually help prevent food allergies. Dr. Sampson recalls that, in 1980, when he started researching food allergies, as a fellow in immunology at Duke University, “I approached the subject with the assumption that I would prove it didn’t exist,” Sampson said. Sampson watched as the incidence of food allergies rose alarmingly in the West while cases remained rare in Africa and Asia.

The hygiene hypothesis.  Observations such as those above have led to an alternative explanation for the mushrooming of modern allergies. The hygiene hypothesis holds that the upsurge in allergies is caused not by an increased exposure to foreign substances, but rather by too little exposure to these allergens, particularly during childhood!  It holds that microbes are our friends; the bacteria and other microorganisms in our gut, on our skin and throughout our bodies protect us from intruding pathogens and foreign substances.  What is more, early childhood exposure to a variety of novel environmental materials — colloquially known as “dirt” — is essential to stimulating, developing and strengthening the immune system. But the modern transition to a more sterile environment has deprived us of both the natural microbial defense system and a mature and robust immune system. The lack of primary microbial and immune defenses leads the body no choice but to activate its emergency backup system: the allergic response.

While modern hygiene and medicine have provided great benefits by controlling or eliminating deadly infectious pandemics like influenza, the plague, and malaria, we may have gone too far with the concept of hygiene.  The current equation of sterility with cleanness or goodness has become a misguided obsession, robbing us of our primary defenses.

The human immune system is complex, and it is impossible to do an adequate job of fully explaining how it works in a short article like this.  One of the clearest, most accessible accounts of the immune system I’ve seen is in a recent book about the hygiene hypothesis: “Why Dirt Is Good: 5 Ways to Make Germs Your Friends”, by Dr. Mary Ruebush.  Ruebush explains that we actually have two immune systems:  the innate immune system and the adaptive immune system. The innate immune system wards off infection from pathogens in a generic, non-specific way, using cells like phagocytes and lymphocytes to physically engulf and remove the invaders. But it is the adaptive immune system that allows us to respond more selectively to very particular invaders without attacking ourselves or “friendly” substances like foods or benign and familiar substances in our environment.  And it is the adaptive immune system that goes awry with allergies, sensitivities…and other immune disorders such as auto-immune disease.  But how does this happen?

Adaptive immunity. Your adaptive immune systems consists of antibody-carrying B cells (produced in your bone marrow) and T cells of two types (Helper T cells and Killer T cell) from the thymus, a small organ in the middle of your chest.  The thymus is like a “school” in which T cells are “educated” to respond to specific external threats.  There is a critical period of the thymus organ in early childhood, during which the adaptive immune system is “educated”; after this critical period the thymus shrivels down in size and becomes less useful.  By means of exposure to a variety of environmental agents during this critical period of our youth, this adaptive immune system puts together an army of specialized “soldiers”:

  • macrophages that roam around as “sentinels” throughout the body to encounter invading antigens;
  • neutrophils that serve as “foot soldiers”, bringing distress messages back to the thymus;
  • helper T cells in the thymus that can recognize the antigen and rapid multiply themselves, sending out legions of reinforcements;
  • killer T cells and B cells, the actual antigen fighters that are directed by the helper T cells.  The B cells go after extracellular antigens, whereas the killer T cells attack body’s own cells that get infected with intracellular antigens (e.g. viruses).

The adaptive immune cells (B and T) cells develop normal responses only if they are stimulated by exposure to foreign substances. Most of the B and T cells remain “uneducated” and have a short lifetime and rapid turnover. Only a very few B and T exposed cells become memory cells with a long life time. Children get primed with IgG antibodies from their mother and IgA antibodies from breast milk which provide “passive” immunity for the first two years of life.  After that, children need to begin activating their own adaptive immunity – their own IgMs and IgGs.

Insufficient exposure means an immature immune system. If this process of educating the adaptive immune system is not sufficiently activated in early childhood, the immune system of the adolescent or adult remains underdeveloped.  Then the response to foreign bodies relies more on the “emergency” system, using IgE antibodies instead of IgG, IgA, or IgM antibodies.

It is these IgE antibodies that tend to overreact, causing allergies. But what is the origin of this strong, misplaced IgE defense? Many immunologists think that the IgE defense most likely originated as a defense against parasites. According to Ruebush,

Parasites are a special problem for your immune system. They are the only category of invader in which the foreigner is actually orders of magnitude larger than anything your immune system has to combat it…Your mast cells hang out just below your mucous surfaces (like in your nose and intestines) and just under the skin. They’re on the lookout for invading parasites…If the mast cells go on the attack against the parasite, they simply explode–they bascially napalm the area. Basophils and eosinophils, the other granulolytic parasite-attacking cells, join in….Allergies are an example of a good immune response gone bad…In developed countries parasitic diseases are no longer common…Overall, this is a good thing…The lack of even a few parasites in your body, however, can be a problem….In the absence of parasites, [the] anti-parasite response can become misdirected against harmless substances in the normal environment. And when your body unleashes an anti-parasite response against something harmless, the damage to your own tissues cause the miserable symptoms of allergies: runny nose, sneezing, hives, diarrhea, and possibly even death by anaphylactic shock.  (WDIG, pp. 79-86)

Essentially, an “under-trained” adaptive immune system, such as that of someone raised in a sterile environment, is more prone to confuse harmless foreign bodies like pollen, dog hair, peanuts, eggs, or insect venom, for parasites. Their IgEs become sensitized towards these allergens, attach themselves to the mast cells on mucous membranes or beneath the skin.  Once the allergen reappears, a full-blown chemical attack, including histamine release, is initiated.

There is a genetic tendency for children of allergic parents to have allergies, but this can be short circuited through breastfeeding. Babies who are breastfed during the first four months have more IgA antibodies, fewer IgE antibodies and a much lower prevalence of allergies.  After breastfeeding, the next most important step is to expose children to a variety of environments, including different foods, flowers, people, animals, and–yes–dirt and “germs”!

So much for scientific explanations. But what can you, as an an adult, actually do if you have problem allergies? Maybe you missed the boat in childhood and didn’t get sufficient exposure to allergens, microbes or — perish the thought — parasites.  But now you are past childhood.  Is there anything you can do to repair the situation, strengthen your primary adaptive immunity, and dampen your “emergency” IgE allergic response?

The answer is: most likely, yes.

Hormetism for allergies.  Readers of this blog have learned about the principle of hormesis, a broadly validated biological principle by which exposure to modest amounts of stress stimulate an organism’s defense and repair processes, resulting in a net increase in the capacity to face increased levels of the original stress.  Hormetism is nothing more than the systematic, practical application of the science of hormesis in order to strengthen your body or mind in one way or another.  Examples of Hormetism discussed on this blog include:

So is there really a way to apply Hormetism to allergies?  The answer is yes and it follows directly from the hygiene hypothesis, which in effect holds that the development of the human system itself is a paramount example of hormesis.

There are two promising approaches that have been scientifically validated.  The first one is quite interesting, but only for the brave.  The second has been applied for close to a century and is undergoing a recent resurgence with an excellent track record of permanently reducing or eliminating allergies.  I’ll start with the more unusual of the two approaches.

Helminthic therapy. This is a nice name for what others have called, more colloquially, “worm therapy”.  It follows directly from the hypothesis that for most of their evolutionary history, humans co-evolved with parasites.  In the modern area, with the virtual elimation of parasites from the Western world, it is this lack of exposure to parasites that has led to an overactive and inappropriate IgE response.

Helminths are nonpathogenic parasites that are deliberately introduced into patients.  There is a body of research suggesting that helminths can be very effective in rebalancing the immune response, moderating the IgE defense, and thereby reducing or eliminating allergies.  I realize this may seem repugnant to some, but the parasites used are microscopic and benign, not the grotesque tapeworms and bizarre organisms seen in medical textbooks. If interested, there is some useful background and references on the Helminthic Therapy website.

Allegen immunotherapy. The conventional medical approach to dealing with allergies is to avoid exposure to allergens (for example by installing air filters and keeping your carpets clean)  and to disable the immune or inflammatory response by medicating with histamines.  As with many applications of Hormetism, allergen immunotherapy takes a diametrically opposite approach. By exposing the patient first to minute amounts of allergen, and progressively increasing the exposure in a systematic manner, the primary adaptive immune system is strengthened. According to Dr. Adrian Morris, the emergency IgE response is dampened by means of stimulating production of an allergen-specific IgG that blocks the IgE response, and possibly also by modulation of the helper T cell response.  In many ways, allergen immunotherapy resembles vaccination, except that object is not to raise antibodies, but rather to normalize an overactive immune response.  Also known as hyposensitization therapy or tolerization, allergen immunotherapy has been found to be very effective:

Allergen specific immunotherapy is the only treatment strategy which treats the underlying cause of the allergic disorder. It is a highly cost-effective treatment strategy which results in an improved quality of life and a reduction in allergic- and allergen-related asthma, as well as a reduction in days off school/work. Immunotherapy has been shown to produce long-term remission of allergic symptoms, reduce severity of associated asthma as well as reduce the chances of new sensitizations to allergens developing… The benefits of allergen specific immunotherapy are long lasting unlike symptomatic based treatments. Immunotherapy is most effective for pollen, dust, and animal dander allergies, and may help those with asthma.

Lest you think that exposure to an allergy-provoking allergen would be dangerous or set off your allergy, read on:

Even the most allergic individual can tolerate minuscule amounts of an allergen without experiencing symptoms. Immunotherapy commences with the subcutaneous injection of a tiny amount of offending allergen, and gradually increases the dose until the individual’s immune system is essentially ‘retrained’ to tolerate exposure without producing an allergic response. This process is also known as specific immunotherapy.

Clinical methods. The most established method of adminstering allergen immunotherapy in the U.S. is to give allergy shots, starting very dilute, and gradually increasing in potency.  There is an emerging approach that appears to be more effective, faster, and has fewer side effects: sublingual immunotherapy, also known as SLIT.  Minute amounts of the allergy provoking substances are formulated in “allergy drops” that are placed under the tongue. While SLIT is practiced widely in Europe, it is still being evaluated by the FDA and is not yet approved in the U.S.

With both allergy shots and subligual therapy, treatment is usually started a few months before the start of allergy season, to build up tolerance. Typically, treatment must be continued for 3-5 years to be fully effective, and may need to be periodically repeated to maintain tolerance.  If interested, you may want to check out several additional references that give a balanced appraisal of allergen immunotherapy, including this 2000 study in the journal Thorax, and this overview in  WiseGeek which suggests that children in particular may benefit from this approach.

Other applications of immunotherapy. In the introduction, I suggested that immunotherapy may also be effective in treating other immunological “overreactions” including chemical and food sensitivities and auto-immune disorders. There is less evidence and research in these areas, and in order to keep this article short, I’ve chosen not to address these possibilities.  But I think it is worthwhile to follow the research in this area.  If the hygiene hypothesis is true, which I think it is, then the possible applications of “exposure” therapies are numerous and offer great promise.

Share your thoughts on this topic on the Discussion Forum.



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  1. Bob

    In my family, on my mum’s side, almost all of the young second/third born kids have some kind of food/skin allergy. DrCate has also noticed this possible pattern.

  2. jcrestmont

    Another great post, Todd! My mother raised me with the motto “dirt is good” and we were always getting covered with dirt playing outside. Nobody in our family has allergies. I notice that a lot of children these days bring inhalers to school, and their homes are typically immaculate. So I thought there had to be a connection. And now you’ve shown us the science.

  3. David I

    I think there is something to the exposure theory.

    On the other hand, from the time I was very young I was allergic to cats and cut grass–and it wasn’t as though I lacked exposure to either! The allergies have faded as I’ve grown older, but a few days in a cat-saturated house can still get to me.

    The body undoubtedly adapts to low-level exposure to irritants, and I would like to think that it adapts in a healthy fashion–perhaps by learning not to respond rather than just responding less. But some people think that ongoing low-level exposure to irritants puts the body in a state of low-level inflammation–and, as I’m sure you know, low-level inflammation is the flavor of the week in causing everything from cardiovascular disease to cancer. So this is an interesting issue.

    As to the gut flora of Italian vs African children, I think it is possible that the researchers are putting too much blame on diet. I would guess that the major difference is antibiotics!

    • Todd


      I think you are right on the money regarding the difference between chronic low-level inflammatory irritants (bad for the immune system) and intermittent exposures that begin early in life and build up tolerance (good for the immune system). You are also right that the study of Italian vs. African children and their intestinal flora is just one data point, with alternative possible explanations. By itself, that study proves nothing. But in combination with a broad range of other observations, I think it helps to make the case for the hygiene hypothesis.

      Regarding your childhood exposure to cats and cut grass: As I understand the hygiene hypothesis, the critical phase for developing a strong, non-allergic immune system is very early, within the first two years of life. Breast-feeding and exposure to a variety of foods and environmental agents is critical during that period, exposure after two years has less impact. It would be interesting to know whether you were breast-fed, and what your early diet and environmental exposure were like. Of course, it may turn out that you were breast-fed, etc. and still have allergies, and the converse may be true. These are not 100% correlations; there is always the role of genetics and other factors. But I think the effects of early exposure may explain a lot.

      As to what you can do about it now, I looked into this and learned that a fast-acting cat allergy vaccine called Allervax Cat was developed in the 1990s by Immulogic and clinical trials at Johns Hopkins and Stanford were extremely promising. However,the cat allergy vaccine project was halted after Immulogic was acquired by Merck, and it was sold to a succession of companies over the last decade. The cat vax is now being trialed by Pharmaceutical Corporation in Cambridge, Mass. Until the vaccine is available, you could always get traditional allergy shots for cats and cut grass, prepared by an allergy doctor.


      • David I

        Yep, I was breast-fed, and for what some people thought was an unusally long period of time. And I was certainly exposed to both cats and grass from the earliest possible age–I grew up in a relatively rural setting in Southern California.

        On the other hand, I nearly died of pneumonia in my first year of life. I think violent infections can cause all manner of collateral sensitizations in infants; anything around when the developing immune system is on full alert is likely to be treated as a guilty party!

  4. Gina

    Aloha Todd!
    I just now heard your interview with Jimmy Moore and am thrilled to have found your site!
    Your views ring true with me and the clients I see. As a new reader I will be immersing myself in your posts. Thanks for sharing your knowledge and expertise. I’ll be a regular here I’m sure.
    Much Aloha, Gina

    • Todd

      Glad you found the site, Gina. I look forward to your contributions. — Todd

  5. Ah, this topic is near and dear to me, as I have a six-year-old with seasonal allergies. He also has some mild food allergies, several food sensitivities, and sensory processing issues. He was breast fed, has been an excellent eater, great at trying new foods, and was always an outdoorsy child whose favorite activity still is digging in the dirt. Meanwhile, my five-year-old was only breastfed half the time, much prefers playing with the computer to dirt, and has a very limited, “fussy” diet, but has no allergies or sensory processing problems whatsoever. Go figure.

    We are looking into a form of exposure therapy, though of the mental kind – neural linguistic programming. Apparently, it has been used for quite some time now to treat allergies. We’ll see.

    I confess, I’m leery of allergy shots for little kids…

    I have heard of worm therapy to treat gluten intolerance issues – pretty interesting!

    • Todd


      Well, it looks like your experience with your two children is not very well explained by the hygiene hypothesis or Dr. Ruebush’s claims about breastfeeding. I’m always interested in “exceptions to the rule” like yours, because they can help us to expand and improve the theory. Beyond breastfeeding per se, I’m interested in whether your more allergic youngster was introduced to a variety of solid foods very early on (at 6-12 months) or whether you delayed or avoid this until later. Dr. Sampson, who I quoted from the New Yorker article above, found that food allergies in young children, e.g. milk allergies, could be reversed by very gradually introducing very dilute amounts of the food allergen. Often cooking, baking, or heating the food at first, to denature the protein, helped to get the process started.

      I’d be interested to find out if NLP helps your child’s allergies. I’m a bit skeptical about a “psychological” cure for allergies. But on the other hand the there is a respected field called “psychoneuroimmunology” based on well established connections between the nervous system and the immune system. So I’d love to hear more.


  6. Jim

    Love this blog and I can now stay in the cold shower for several minutes. I got rid of all my allergies and hay fever after going low carb and wheat/grain free. I think a leaky gut is the cause.

  7. Todd, I introduced my older son to solids in a sort of haphazard way – for example, I gave him wheat and dairy before 12 months, and shortly after that he developed eczema. I removed these foods (both at once, so never clear on which was the problem, if either) and the eczema cleared, and when I reintroduced the foods months later he seemed fine, but as a preschooler, I noticed problems (more with food additives, but also with natural chemicals like salicylates, and possible gluten and dairy sensitivities, but hard to be sure). We’ve had good results with food enzymes that target wheat and dairy proteins, as well as phenols, and my son now pretty much eats anything in moderation, with the exception of artificials (which I am happy to keep us all off anyway). Perhaps the enzymes help in a way similar to the minute reintroductions, or to the cooking and denaturing of proteins?

    His only “true” (IgE) food allergy is a pretty mild one to peanuts (no reactions, but it showed up on a blood test). His doctor suggested we avoid them, which I suspect may be a mistake, but we’re doing so for now.

    I have this unrealistic hope that now that we have food pretty well sorted out, he’ll have fewer issues with seasonal allergies…this spring will be the test. There are some cross issues I know with seasonal allergies and foods. We’ll see. I have been giving him raw local honey, just in case it can help with desensitizing.

    I will certainly let you know if we pursue NLP and if anything interesting happens! Thanks for this interesting blog and forum. I’ve been talking up cold showers, and am now the family eccentric. (Well, I already was…)

    • Todd


      Glad to hear that your son’s food allergies seem to be moderating to some degree. My understanding is that most people grow out of their allergies as they get older — as long as we allow that to happen and don’t overmedicate with antihistamines or other allergy medication! Assuming his allergy to peanuts and seasonal allergies are not severe, you might discuss the idea of gradual exposure starting with minute amounts, following Dr. Sampson’s approach. (You might find a copy of the recent Feb. 7, 2011 New Yorker article and give it to his doctor). For example, you might grind up a single piece of peanut to give to him in a meal. If that produces no reaction, then gradually go to two peanuts, etc. Again, these are just ideas to discuss with his doctors and your family — I’m not a doctor and don’t pretend to give medical advice.

      As far as being eccentric goes, my daughter recently gave a talk in high school speech class about “my unusual parents”. Although from what I heard, at the end of the talk she admitted that she wouldn’t have it any other way.


  8. Todd, interesting article, thank you!

    Along these lines, I know a child who had a severe peanut allergy as a toddler (anaphylactic reaction), so his parents naturally avoided all peanuts and carried an epi pen. After a couple years, a blood test indicated that the boy was not nearly as allergic any more, so they tested him (in a hospital, of course!) and he had no reaction. So his “job” these days is to eat peanuts at least once very four days, the idea being this regular exposure will keep him “unallergic”. (I doubt the parents have given up the epi pen just yet, though!)

  9. JO

    Hi, I’m just wondering and trying to learn more regarding cat allergies. I have two little boys aged 6 and 3 and they are both very healthy children. no allergies, that I know of, and both neither me or my partner or any of our families have allergies or asthma. Both my children have been bought up around animals as I adore animals and think it good for them. It’s goo educational wise, and my love for animal has played a part of good grades for my oldest son, who working at school above the national educational level in science. However, we have just purchased another kitten it’s a Siamese and we all adore him. I have recently found out via Internet sources and my own doctor that children can develop pet allergies regardless of how long they have been living with pets. is this very common or is it more common if the child has an allergy to cats when they first get a new pet at an older age ? I’m confused ? How can a child just all of a sudden develop an allergy to cat when they have lived with them for a number of years? My children haven’t shown any symptoms of allergys, however I’m quite worrie tht they will ? Do I need to worry ? Do almost all children have a reaction to Cats ?

    • Todd

      Hi JO,

      I’m not an allergist or immunologist, so I really can’t predict whether or not your kids are susceptible to developing specific allergies to cats. But as a general rule, I think you’ve done the right thing by raising your children with close contact to pets from a young age. As long as they are otherwise healthy, I think their exposure to animals — and to dirty things in general — helps to develop a strong adaptive immune cells, with strong IgM and IgG response, and therefore less likelihood of an over-reactive IgE (allergic) response.

      Certainly it is NOT the case that almost all children react allergically to cats. In fact nobody in my immediate or extended family has allergies, and we have always had cats and dogs and never stressed cleanliness.



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