Determinants of Clinical Allergic Disease
The Relevance of Indoor Allergens to the Increase in Asthma

THOMAS A. E. PLATTS-MILLS, KEVIN BLUMENTHAL, MATTHEW PERZANOWSKI, and JUDITH A. WOODFOLK

Division of Asthma, Allergy, and Immunology, Health Sciences Center, University of Virginia, Charlottesville, Virginia

	INTRODUCTION

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Over the last 40 years of the twentieth century, some remarkable changes have occurred in the prevalence and severity of allergic disease (1). Understanding the reasons for the increases is important both in terms of management but also in understanding the etiology of the diseases. In the United Kingdom and Germany the published studies suggest that the increase in asthma is part of an overall increase in skin test reactivity (2) or of allergic disease (3). In contrast, in the United States there is little evidence of an increase in hayfever over the last 30 yr, and it appears that there has been a selective rise in asthma (4, 5). Similarly, in comparing Guang Chou in southern China with Hong Kong, Leung and Ho found similar prevalence of skin test reactivity, but a much higher (and increasing) prevalence of asthma in Hong Kong (6). A general rise in allergic disease could be explained by an increase in immune responses of the helper T cell 2 (Th2) type. In contrast, a selective rise in asthma would best be explained by either an increase in an exposure that selectively affects the lungs or by a progressive loss of some factor that protects the lungs from wheezing.

When Coca first coined the term "atopic" he included those diseases in which a family history and positive skin tests were common, that is, atopic dermatitis, hayfever, and extrinsic asthma. The allergens that are relevant to these diseases are now well defined. It is also clear that the different allergic diseases are associated with different groups of allergens. Seasonal rhinitis or hayfever is strongly associated with positive skin tests to allergens derived from pollens and outdoor fungi. In contrast, in most studies the dominant allergens associated with asthma are present in the environment all year. In general, this means indoor allergens, for example, dust mite, animal dander, cockroach; however, in several areas of the world the fungus Alternaria is also significantly associated with asthma (7). Thus, the important characteristic of the allergens associated with asthma may be exposure for many months of the year rather than the site of exposure. Multiple different proteins and glycoproteins are involved, including molecules that have diverse biological roles, and not surprisingly, many of the defined allergens have extensive homology with known enzymes (16). While it is possible that enzymatic activity plays a role in the immunogenicity of these proteins, there are several reasons for doubting this role. In general the putative enzymes have not been shown to have enzymic activity under the circumstances prevailing in the respiratory tract (17). Most studies have not found any evidence that allergens have an effect on nonallergic individuals. At least two allergens, which are important in asthma, have no homology with known enzymes: the group 2 mite allergens (Der p 2 and Der f 2) and Fel d 1 from the cat. At this time the idea that enzymatic activity of allergens plays an important role in immunogeneity should be regarded as a hypothesis that still lacks direct support (18, 19). The main characteristics of an allergen are that they are immunologically foreign, freely soluble, and are inhaled repeatedly.

	THE IMMUNE RESPONSE TO INHALANT ALLERGENS

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In 1935 Cooke and colleagues established that the serum of allergic individuals had two different kinds of antibodies: skin-sensitizing antibodies and blocking antibodies. Subsequent studies reestablished that the response includes IgG and IgA antibodies, as well as the IgE antibodies, which bind to the high-affinity receptor on mast cells and basophils. Once it became possible to study specific T cell responses, it was found that allergic individuals gave proliferative responses to purified allergens whereas most nonallergic individuals did not (20, 21). Subsequent studies established that the allergen-specific T cells were predominantly CD4⁺ and produced cytokines characteristic of Th2 cells (22). In some cases it has been possible to clone allergen-specific T cells from nonallergic individuals, and these were found to have a different cytokine profile (24). This evidence has been taken as support for the view that the normal or nonallergic response to allergens is a Th1 response. Indeed, this argument has been taken further to argue that allergy represents an abnormal persistence of the neonatal Th2 response (25). There are, however, major problems with accepting the view that normal individuals have made a Th1 response. First, inhalant allergens are not associated with delayed hypersensitivity skin test responses. Second, the in vitro T cell responses of nonallergic individuals to mite or cockroach allergen are generally small (i.e., stimulation index [SI] < 3) and are not comparable to those found with Th1 responses to tetanus, Trichophyton, Candida, or PPD (26). Third, there are no examples of the major indoor allergens (e.g., mite, cat, or cockroach) giving rise to lung disease such as hypersensitivity pneumonitis that could be associated with a Th1 response. Finally, the antibody response in nonallergic individuals is not typical of a Th1 response; high-titer IgG₁ antibodies are unusual; precipitins to common allergens are rare; and many of the sera that have IgG antibodies also have IgG₄ antibodies (27, 28). The presence of IgG₄ is significant because this isotype is dependent on Th2 cytokines. Vercelli and colleagues have demonstrated that the expression of IgG₄ is dependent on interleukin 4 (IL-4) (29). An immune response that includes IgG₄, with or without IgE, is probably dependent on a Th2 response. Although there may be T cells with a Th1 phenotype in some nonallergic individuals, these cells are not sufficient to give proliferative responses in most patients; and there are no delayed-type hypersensitivity (DTH) skin responses, no precipitins, and no disease typical of Th1. The correct conclusion is that in most cases the nonallergic state is either a nonresponse or a modified Th2 response.

	DETERMINANTS OF AN EARLY RESPONSE

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The human fetus is capable of making immune responses from the beginning of the third trimester. Indeed, there is clear evidence that some infants are allergic to food antigens when they are born or become allergic during the first few months of life. It is also possible that a response to inhalant allergens occurs in utero or infancy. However, the quantities of an inhalant allergen such as dust mite reaching the fetus must be low indeed, and there is no consistent evidence to support the hypothesis that sensitization occurs in utero. The most widely cited phenomenon is the proliferation of cord blood T cells in response to mite allergens in vitro (30). While the exact significance of this response is not clear, the evidence available does not support the idea that this proliferation represents an immune response. First, the response is transient and is not found at 6 mo. Second, other groups find that the response is less common than originally described and that it has no predictive significance. (A. B. Becker, personal communication, 1999; and Reference 33). Finally, the response does not appear to be influenced by exposure of the mother. Thus, in Sweden mothers who are exposed to less than 1 µg of Der p 1 per gram of dust still have babies whose cord blood T cells divide when exposed to mite extract (31). Some groups have reported evidence of antibody production to common inhalant allergens in early life. These results have been obtained by enzyme-lined immunosorbent assay (ELISA), which is susceptible to nonspecific binding, and tends to overdetect low-affinity antibody (34). Using radioimmunoprecipitation assays there is no nonspecific binding from IgG, and because radiolabeled antigen is bound in the fluid phase it requires high-affinity antibody (27, 35). These assays can be modified to measure other isotypes including IgE and IgG₄ (27). In a prospective study in England most of the children had no detectable antibody before age 4 yr. At age 2 yr we could identify only one-quarter of the infants who subsequently became allergic. After age 2 yr the children who were going to become allergic developed IgG, IgG₄, and IgE antibodies to Der p 1 in parallel (7, 27). At present the hard evidence about the immune response to inhalant allergens does not support an event in utero or in early infancy and it is much more likely that the response occurs over the first 4 yr. It is patently absurd to refer to universal sensitization to dust mite antigens in utero on the basis of the currently available evidence (36).

Sensitization of infants in utero would have to be influenced by the immune status of the mother, because passage of allergen across the placenta would inevitably be influenced by maternal antibody. In support of this concept it has been reported that the mother's history has more influence over the development of asthma in early childhood than the father's history of asthma. However, this influence of the mother is only over early wheezing and not over allergic responses or asthma developing after age 4 yr. At age 10-13 yr the parents have equal genetic influence over allergy and asthma.

	FACTORS INFLUENCING THE CLINICAL OUTCOME

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Given the strength of the epidemiological association between immediate hypersensitivity to indoor allergens and asthma, it is logical to assume that allergen exposure plays a role in the disease. In fact, there is a wide range of experimental evidence supporting the view that inhaled antigen is a contributor to the inflammation and symptoms of asthma, if not a primary cause of the disease (37). The evidence can be focused in three areas.

1. The association with sensitization to indoor allergens is strong, has been consistently found in many parts of the world, and generally reflects the allergens found in the average house in the community (38).

2. Bronchial provocation with allergen can produce immediate and late responses in the lung. Indeed, provocation of the lung with a relevant allergen is the best established method for recruiting eosinophils into the lung and producing a prolonged increase in bronchial hyperreactivity (BHR) (39, 40).

3. Moving patients with asthma away from their homes to sanitoria or hospital rooms has consistently resulted in decreases in symptoms and decreases in BHR (38, 41). In addition, the controlled trials of avoidance that have achieved prolonged decreases in dust mite exposure in patients have also resulted in significant decreases in BHR (38, 42).

Although the evidence that allergens play a role in asthma is convincing, it is an entirely different question whether increased exposure can explain increases in asthma occurring over a 40-yr period. Increases in asthma have been associated with sensitization to dust mite, cat, cockroach, and Alternaria allergens in different communities and in different parts of the world (7). Changes in lifestyle have increased the amount of time spent indoors. In addition, there have been many changes in houses that could have increased exposure to allergens, for example, warmer, lower ventilation, increased furnishings. However, in order to explain the increase in asthma simply by increased exposure it would be necessary to propose that multiple different allergens had increased in parallel. It seems unlikely that changes in mite, cat, and cockroach allergens could have occurred in parallel, or that the changes have been sufficient to explain the scale of the changes in asthma.

The question, then, is what other changes in Western lifestyle could have increased allergic disease? At this point it becomes important to decide whether the increase in asthma has been part of an overall increase in allergic disease. An increase in allergic disease could be explained by a change in immunity. The possible explanations include the following: changes in diet, the introduction of broad-spectrum antibiotics, increased vaccination in early childhood, decreased infections overall, or decreased viral infections in early childhood. There are epidemiological studies supporting each of these possibilities; however, none of them are supported by direct evidence that the changes can cause the disease. Furthermore, the odds ratios associated with these effects, that is, 1.1-1.3, are not comparable to the odds ratios associated with sensitization to indoor allergens (43).

If, as we have argued earlier, the evidence is for an increase in wheezing among allergic individuals then the arguments are different. Increased wheezing could occur because of a selective insult to the lungs or because of the progressive loss of a factor that under normal circumstances protects the lungs from wheezing (Figure 1). The association between indoor exposure and asthma is well-established. What is less well recognized is that indoor exposure is uncommon as a cause of conjunctivitis. Thus, an increase in asthma with a decline in conjunctivitis might have occurred simply on the basis of spending more time indoors. Figures available from managed care organizations in the United States show that a large number of individuals with asthma are also being treated for rhinitis. However, it is not clear whether rhinitis has increased because 16% of the population reported rhinitis as early as 1960. 

View larger version (15K): [in this window] [in a new window]   Figure 1.   Allergen exposure and other factors that could have contributed to the increased prevalence and severity of asthma: 1960-1990. 1 = Increrased mite or mold growth; 2 = increased airborne animal dander; 3 = increased cockroach growth; 4 = can enhance response to allergens.

The possibility that sedentary lifestyle is harmful to the lungs has received little attention until recently (44). Children today are spending as much as 3 h/d in front of video screens, televisions, and computers (45). The evidence in the Surgeon General's report about increasing obesity in the United States is also best explained by decreased physical activity (46). Luder and coworkers reported in 1998 that asthmatic children in New York are heavier than their counterparts in that city (47). Thus, it appears that the decline in physical activity is at least as severe among those children who have asthma as it is in the community in general. Sedentary lifestyle could influence the lungs in several ways. It has long been recognized that full expansion of the lungs can decrease lung resistance, and that patients with spinal cord injuries have BHR. Skloot and colleagues have demonstrated that relatively short periods of time spent without taking a deep breath can make the lungs of a normal individual hyperreactive to methacholine (48). Fredberg and colleagues in Boston, Massachusetts have provided a mechanism for the bronchodilator effects of full lung expansion. They have shown that without regular extension smooth muscle will start to contract at a shorter length. They went further to speculate that a decline in spontaneous sigh rates would give rise to BHR (49). Prolonged periods spent in any sedentary activity will decrease full expansion of the lungs. However, we have shown that spontaneous sigh rates are significantly lower while watching a screen compared with reading (50). Thus, some passive visual activities might decrease sigh rates below a critical level. The alternative hypothesis would be that prolonged physical activity (such as children's play) is beneficial to the lungs either physiologically or via an effect such as accelerated healing of allergen-induced inflammation (17). Clearly direct experiments on the effects of exercise on the lungs of individuals with asthma are needed.

	CONCLUSION

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In some Western countries the increase in asthma is such that as many as three-quarters of the children who are currently presenting for treatment would not have had symptoms in 1950. Given these figures it is clearly important to identify the causes of the increase. The main theories can be divided into three types, which may well overlap (17). The most striking fact about the epidemiology is that sensitization to indoor allergens remains as strong a risk factor for asthma today as it was in 1970 when increases were first reported. Thus, it is obvious that the increase is primarily among allergic individuals. Nonetheless, increase in allergen exposure is not a convincing explanation of the progressive rise in asthma associated with so many different allergens. For this reason several theories have been developed about factors that could have increased immune responses or decreased the threshold for wheezing among allergic individuals. In the United States asthma has increased as judged by treatment, national surveys, hospital admissions, and mortality rates. The "epidemic" in the United States is striking because it is worst among individuals living in poverty, and particularly African Americans living in poverty (14, 15, 51). In most other countries in the Western world asthma is not associated with poverty. In fact, many of the theories proposed to explain the increase in wheezing in Europe and Australia cannot explain what has happened in the American cities. Thus crowded and poor housing remain common in the big cities; furthermore, vaccination rates are generally lower and infection rates are generally higher. The rise in obesity among children, which is appearing in many countries, is most marked among poor children in the United States. Thus, decline in physical activity must be considered as a possible factor contributing to the severity of asthma in American cities. As we have seen, this may interact with high exposure to allergens such as dust mite and cockroach and high levels of sensitization. While it is possible that risk factors for asthma in the United States are different from those in other countries, it seems more likely that the progressive increase in asthma that has occurred over the period 1960-1995 in so many communities has common causal elements.

	DISCUSSION

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Holt: The important take-home message with respect to human immune responses to inhalant allergens is that these Th1 responses are not large. They are small and nobody is even remotely suggesting that they should cause DTH. It takes more than just having cells that make IFN- when you challenge them to get DTH. I don't think that the work is leading us astray. We have the technology to measure these low-level responses. I should also point out that a lot of this work is being done by looking at epitope mapping, so we are down to a level of specificity that takes us beyond any discussion about mitogens.

Platts-Mills: Why do you think that among all the immune individuals not one develops a full Th1 response, i.e., with high-level proliferation and IFN- production and DTH?

Holt: When you put BALB/c or C57B6 mice on a chronic aerosol exposure protocol they make the default immune response, which is Th2, for a few weeks; they make IgE. Then they lock into a much more Th1 profile and you can show that you get T cells that make IFN-, etc. They lock into a midrange level of a Th1-dependent IgG subclass production. They never, ever get DTH. You can challenge these animals and they will not get secondary immune responses. They have locked into a certain level. It is not due to antibody feedback, because you cannot transfer that with serum, but you can transfer it with T cells. So, this negative feedback self-limits that response. Something like that also happens in humans. There is some sort of feedback within the T cell response.

Platts-Mills: Are you going to say the same about the IgG₄ situation? IgG₄ is totally IL-4 dependent and should definitely be taken as a marker of a Th2 response.

Holt: Yes, but it is less sensitive to, for example, IFN- effects than IgE is. What seems to happen is that you can take out the most IL-4-dependent response first, which in this case is IgE and you come down the line and leave other isotypes.

Platts-Mills: These T cell responses that you report disappear at 6 months and no IgG antibody starts until age 2. Yet, you are proposing that everybody gets sensitized in utero.

Holt: No, not sensitized; weak priming occurs in many individuals. Sensitization is different.

Platts-Mills: That is semantics. Does this "weak priming" have any relationship to subsequent events?

Holt: We don't know what the answer is. It has been published by some groups, for example, that it is pathogenic to be primed in utero. We don't believe that for one minute. Our belief is that this is part of normal biology. It is part of the immune system learning what is in the environment. We don't know how it learns not to go too far in the Th2 direction. I also want to point out that there are many other groups, not just us, who are reporting these allergen-specific responses. Only two of the groups that I know are using AIM-V, so it is not something magic in the culture system.

Platts-Mills: Dr. Yeung has clearly shown that these proliferation responses occur in cord blood, repeating what your data shows. However, at 2 years they find no relationship to outcome. Maybe they are not measuring the right thing yet, but at the moment I am not aware of any data showing that proliferation responses in cord blood predict outcome that is relevant for atopy.

Holt: Nor do we.

Björkstén: There are several studies now comparing East and West Germany, Estonia, and Sweden, showing that the major differences in the prevalence seem to be limited to the cohort born in the late 1950s or around 1960. If you are asking for something global, around the late 1950s we started to have freezers. I don't think it has to do with antibiotics. It has to do with the fact that a loaf of bread baked in my home is moldy within a week, while I can buy a loaf of bread in the supermarket and it stays in the plastic bag for several weeks without ever getting moldy. It has not got any toxins or poisons in it. It is just that it is a very nice and clear hygiene. The piece of meat that we ate when we were young had many more bugs than the frozen meat taken from the fridge.

Platts-Mills: If you assume it has something to do with cooking behavior: we have in the USA a wonderful range of different populations with staggeringly different behavior and something should come out. All our grain supplies are treated with pyrimiphos methyl, so we no longer have mites in them, so we don't eat mites in early childhood. There are all sorts of different things that have changed, but in most cases those all happened to a population group at one time, whereas the increase in asthma is steadily going up. It is very disturbing that it is still increasing in the nineties.

Björkstén: I am not saying that this is the only explanation. I think that the dietary discussion has to be widened beyond talking about trans-fatty acids or antibiotics. It is really an ecological effect that we are seeing. We should remember that the increase in allergy apparently started to take place in the middle- and upper upper-class population in our countries. One of the strongest relationships in the ISAAC study is the McDonalds index: the more McDonalds outlets you have in a country the higher is the prevalence of the symptoms. I think it is an indicator of something that is changing in the dietary habits in the country.

Platts-Mills: Remember that during the riots in Los Angeles the only buildings that did not get burned were the McDonalds.

Martinez: If we were to put people living in Charlottesville, Virginia, where the main allergen associated with asthma is house dust mites, in a totally house dust mite-free environment by spraying daily and uninterruptedly with these things that you tell us we are now spraying on our grain, would there be zero asthma in Charlottesville?

Platts-Mills: No. Despite the increase in asthma prevalence, the relationship with allergy among children in the age range of 13-18 has stayed just as tight. Your own data has now become very tight: 92% of your wheezing children at age 13 are atopic. So, whatever it is we are looking at, is acting on a lung that is part of an allergic person. If you completely removed allergen exposure, would that reduce it? And the answer is: when children have been taken to Davos, the Davos studies done by Kerrebijn in 1970, 50% of the children became effectively normal, apparently just by being removed from their houses. But lots of other things happened to those children. The experiments were definitely not reducing every allergen to zero and you clearly cannot do that because there are plenty of other sources in the environment.

Martinez: My question is different. Children from parents born in New York who are house dust mite positive who decide to live in Tucson (without house dust mites, or very few) get sensitized to Alternaria. So, the point I am trying to make is, asthmatics get sensitized to what they see where they live. You cannot say that because people get sensitized to this allergen, they get asthma. I agree with you that something makes them become sensitized to perennial allergens on the one hand. On the other hand, this factor that makes them develop asthma, is what causes asthma. I do not discuss whether it is bad for you to become sensitized to perennial allergens, I don't discuss that.

Platts-Mills: May I take your model back, and apply it to the situation of Charleston, South Carolina, in 1960, where there were children becoming sensitized, but there was much less asthma. So, something different has happened to increase asthma. I take what you are saying in the sense that it is no longer possible to believe that it is a simple dose-response. However, everything we know says that this inflammation has to be started by something. The only way we know to start it is by putting allergen into the lung. We also know that in each situation there is a perennial allergen, but that is not the same as saying that if you took that allergen away you know what would happen.

Martinez: People who get sensitized in rural Africa or in China have zero asthma. So, there has to be something (that has to be associated with that sensitization) that is different from that of black kids in Charlottesville. There has to be something different. There are two possibilities. Either the mechanism of sensitization is different, or there is something in the lungs that has changed, that makes it different. What I am proposing to you is a common hypothesis: There is something in the lungs that has changed and makes, on the one hand, the lungs more susceptible to a lot of things and, on the other hand, makes the lungs more susceptible to becoming sensitized, which of course is not good for you, but is not the cause of your disease.

Platts-Mills: There is no question that I am now proposing that there has to be some change in society that has allowed the lung to express this phenomenon (i.e., wheezing) which it did not express before. But the epidemiology remains linked to IgE. Until someone shows epidemiology not related to IgE, this remains the fallback position.

Holt: I have a question on the dose-response in the inner-city American study. I remember from a talk you gave a couple of years ago one of the important points you were making was, that the indoor environment there is almost unique in that it has very high levels of three potent allergens: very high mite, very high cockroach, and also quite often rodent urinary protein. Is that true, or is the dose-response really not that bad?

Platts-Mills: There is very little convincing evidence for a dose-response relationship between exposure and increase in asthma symptoms within a population.

Djukanovic: What about the determinants of asthma severity; how strong is the link to atopy and IgE there?

Platts-Mills: If you look at teenage children admitted to the hospital in Poole, UK, the prevalence of atopy rises with bronchial hyperreactivity, it does not fall. Then there is Dr. Beasely, who stated about the children who died in New Zealand that they were "100%" allergic to dust mite. There is no data in the school age years that the most severe cases are less atopic than the others. In adults we have an extraordinarily complex situation.

Djukanovic: Is the level of specific IgE correlated with disease severity?

Platts-Mills: No. People have looked at that in adults and tried to see that there was some level of IgE antibody that would correlate with severity and that does not happen.

Weiss: The analogy that I gave you was coronary disease: a change in physical activity, obesity, diet and all of them independently predict coronary disease and all of them may independently predict the changes in the asthma epidemic. Now, if you combine that with some global change in the microflora related to food processing or animals or what have you, then you have a very complicated picture. It seems unlikely to me that we are coming out of this with a single exposure that would explain the asthma epidemic.

Platts-Mills: I like the coronary model. Bad diet is only causal in concert with declining physical exercise. I agree, but it could be that one factor (i.e., exercise) plays an important role in asthma when combined with others, which includes being allergic to common indoor allergens.

Kips: If we think that lack of exercise explains at least part of the allergen-driven increase in asthma, have there been any studies looking at the airway behavior?

Platts-Mills: Togias and his colleagues showed that if you did not take a deep breath for 20 minutes you develop bronchial hyperreactivity (49).

	Footnotes

Correspondence and requests for reprints should be addressed to T. A. E. Platts-Mills, M.D., Health Sciences Center Division of Asthma, Allergy, and Immunology, University of Virginia, Box 225, Medical Center, Charlottesville, VA 22908. E-mail: tap2z{at}unix.mail.virginia.edu

	References

TOP INTRODUCTION THE IMMUNE RESPONSE TO... DETERMINANTS OF AN EARLY... FACTORS INFLUENCING THE... CONCLUSION DISCUSSION REFERENCES

1. Woolcock, A. J., and J. K. Peat. 1997. Evidence for the increase in asthma worldwide. In Ciba Foundation, editor. The Rising Trends in Asthma. John Wiley & Sons, Chichester. 122-139.

2. Von Mutius, E., F. D. Martinez, C. Fritzsch, T. Nicolai, C. Roell, and H. H. Thiemann. 1994. Prevalence of asthma and atopy in two areas of West and East Germany. Am. J. Respir. Crit. Care Med. 149: 358-364 [Abstract].

3. Butland, B. K., D. P. Strachan, S. Lewis, J. Bynner, N. Butler, and J. Britton. 1997. Investigation into the increase in hay fever and eczema at age 16 observed between the 1958 and 1970 British birth cohorts. Br. Med. J. 315: 717-721 [Abstract/Free Full Text].

4. Broder, I., N. W. Higgins, K. P. Mathews, and J. B. Keller. 1974. Epidemiology of asthma and allergic rhinitis in a total community: Tecumseh, Michigan. IV: Natural history. J. Allergy 54: 100 .

5. Nathan, R. A., E. O. Meltzer, J. C. Selner, and W. Storms. 1997. Prevalence of allergic rhinitis in the United States. J. Allergy Clin. Immunol. 99: 5808 .

6. Leung, R., and P. Ho. 1994. Asthma and atopy in three Southeast Asian populations. Thorax 49: 1205-1210 [Abstract].

7. Sporik, R. B., S. T. Holgate, T. A. E. Platts-Mills, and J. Cogswell. 1990. Exposure to house dust mite allergen (Der p I) and the development of asthma in childhood: a prospective study. N. Engl. J. Med. 323: 502-507 [Abstract].

8. Peat, J. K., E. Tovey, B. G. Toelle, M. M. Haby, E. J. Gray, A. Mahmic, and A. J. Woodcock. 1996. House dust mite allergens. A major risk factor for childhood asthma in Australia. Am. J. Respir. Crit. Care Med. 153: 141-146 [Abstract].

9. Sporik, R., J. M. Ingram, W. Price, J. H. Sussman, R. W. Honsinger, and T. A. E. Platts-Mills. 1995. Association of asthma with serum IgE and skin-test reactivity to allergens among children living at high altitude: tickling the dragon's breath. Am. J. Respir. Crit. Care Med. 151: 1388-1392 [Abstract].

10. Halonen, M., D. A. Stern, A. L. Wright, L. M. Taussig, and F. D. Martinez. 1997. Alternaria as a major allergen in children raised in a desert environment. Am. J. Respir. Crit. Care Med. 155: 1356-1361 [Abstract].

11. Perzanowski, M. S., R. Sporik, S. P. Squillace, L. E. Gelber, R. Call, M. C. Carter, and T. A. E. Platts-Mills. 1998. Association of sensitization to Alternaria allergens with asthma among school age children. J. Allergy Clin. Immunol. 101: 626-632 [Medline].

12. Ronmark, E., B. Lundback, E. Jonsson, and T. A. E. Platts-Mills. 1998. Asthma, type-1 allergy and related conditions in 7- and 8-year-old children in northern Sweden: prevalence rates and risk factor patterns. Respir. Med. 92: 316-324 [Medline].

13. Sears, M. R., G. P. Hervison, M. D. Holdaway, C. J. Hewitt, E. M. Flannery, and P. A. Silva. 1989. The relative risks of sensitivity to grass pollen, house dust mite, and cat dander in the development of childhood asthma. Clin. Exp. Allergy 19: 419-424 [Medline].

14. Call, R. S., T. F. Smith, E. Morris, M. D. Chapman, and T. A. E. Platts-Mills. 1992. Risk factors for asthma in inner city children. J. Pediatr. 121: 862-866 [Medline].

15. Rosenstreich, D. L., P. Eggleston, M. Kattan, D. Baker, R. G. Slavin, P. Gergen, H. Mitchell, K. McNiff-Mortimer, H. Lynn, D. Ownby, and F. Malveaux. 1997. The role of cockroach allergy and exposure to cockroach allergen in causing morbidity among inner-city children with asthma. N. Engl. J. Med. 336: 1356-1363 [Abstract/Free Full Text].

16. Stewart, G. A., and P. J. Thompson. 1996. The biochemistry of common aeroallergens. Clin. Exp. Allergy 26: 1020-1044 [Medline].

17. Platts-Mills, T. A. E., L. M. Wheatley, and R. C. Aalberse. 1998. Indoor versus outdoor allergens in allergic respiratory disease. Curr. Opin. Immunol. 10: 634-639 [Medline].

18. Hewitt, C. R. A., A. P. Brown, G. D. Hart, and D. I. Pritchard. 1995. A major house dust cleaving CD23. J. Exp. Med. 182: 1537-1544 [Abstract/Free Full Text].

19. Schultz, O., H. F. Sewell, and F. Shakib. 1998. Proteolytic cleavage of CD25 by Der p 1, a major mite allergen with cysteine protease activity. J. Exp. Med. 187: 271-275 [Abstract/Free Full Text].

20. Black, P. L., and D. G. Marsh. 1980. Correlation between lymphocyte responses and immediate hypersensitivity to purified allergens. J. Allergy Clin. Immunol. 66: 394-401 [Medline].

21. Rawle, F. C., E. B. Mitchell, and T. A. E. Platts-Mills. 1984. T cell responses to the major allergen from the house dust mite Dermatophagoides pteronyssinus, antigen P1: comparison of patients with asthma, atopic dermatitis, and perennial rhinitis. J. Immunol. 133: 195-201 [Abstract].

22. O'Hehir, R. E., R. D. Garman, J. L. Greenstein, and J. R. Lamb. 1991. The specificity and regulation of T-cell responsiveness to allergens. Annu. Rev. Immunol. 9: 67-95 [Medline].

23. Romagnani, S.. 1994. Regulation of the development of type 2 T-helper cells in allergy. Curr. Opin. Immunol. 6: 838-846 [Medline].

24. Wierrenga, E. A., M. Snoek, C. deGroot, I. Chretien, J. D. Bos, H. M. Jansen, and M. L. Kapsenberg. 1990. Evidence for compartmentalization of functional subsets of CD4⁺ lymphocytes in atopic patients. J. Immunol. 144: 4651-4656 [Abstract].

25. Holt, P. G., P. D. Sly, and B. Björkstén. 1997. Atopic versus infectious diseases in childhood: a question of balance. Pediatr. Allergy Immunol. 8: 53-58 [Medline].

26. Slunt, J. B., E. A. Taketomi, J. A. Woodfolk, M. L. Hayden, and T. A. E. Platts-Mills. 1996. The immune response to Trichophyton tonsurans: distinct T cell cytokine profiles to a single protein among subjects with immediate and delayed hypersensitivity. J. Immunol. 157: 5192-5197 [Abstract].

27. Rowntree, S., T. A. E. Platts-Mills, J. J. Cogswell, and E. B. Mitchell. 1987. A subclass IgG4-specific antigen-binding radioimmunoassay (RIA). J. Allergy Clin. Immunol. 80: 622-630 [Medline].

28. Blumenthal, K. B., M. J. Hochmair, D. Gold, and T. A. E. Platts-Mills. 1999. Monitoring of isotype specific antibodies in mothers and children from Boston, Massachusetts. J. Allergy Clin. Immunol. 103: S244 .

29. Vercelli, D., L. De Monte, S. Monticelli, C. Di Bartolo, and A. Agresti. 1998. To E or not to E: can an IL-4 induced B cell choose between IgE and IgG4? Int. Arch. Allergy Immunol. 116: 1-4 [Medline].

30. Miles, E. A., J. A. Warner, A. C. Jones, B. M. Colwell, T. N. Bryant, and J. O. Warner. 1996. Peripheral blood mononuclear cell proliferative responses in the first year of life in babies born to allergic parents. Clin. Exp. Allergy 26: 780-788 [Medline].

31. Björkstén, B., B. J. Holt, M. J. Baron-Hay, A. K. M. Munir, and P. G. Holt. 1996. Low level exposure to house dust mites stimulates T cell responses in early childhood independent of atopy. Clin. Exp. Allergy 26: 775-779 [Medline].

32. Prescott, S. L., C. Macaubas, B. J. Holt, T. B. Smallacombe, R. Loh, P. D. Sly, and P. G. Holt. 1998. Transplacental priming of the human immune system to environmental allergens: universal skewing of initial T cell responses toward the Th2 cytokine profile. J. Immunol. 160: 4730-4737 [Abstract/Free Full Text].

33. Smillie, F. I., A. J. Elderfield, G. Cain, F. Y. Patel, G. Tavernier, M. Brutsche, B. Simpson, A. Simpson, A. Custovic, and A. A. Woodcock. 1999. Cord blood lymphoproliferative responses in neonates with defined atopic risk and maternal indoor allergen exposure. J. Allergy Clin. Immunol. 103: S109 .

34. Mariani, R., J. F. Price, and D. M. Kemeny. 1992. The IgG subclass antibody response to an inhalant antigen (Dermatophagoides pteronyssinus) during the first year of life. Clin. Exp. Allergy 22: 29-33 [Medline].

35. Platts-Mills, T. A. E.. 1979. Local production of IgG, IgA and IgE antibodies in grass pollen hay fever. J. Immunol. 122: 2218-2225 [Abstract/Free Full Text].

36. Prescott, S. L., C. Macaubas, T. Smallacombe, B. J. Holt, P. D. Sly, R. Loh, and P. G. Holt. 1998. Reciprocal age-related patterns of allergen-specific T-cell immunity in normal vs. atopic infants. Clin. Exp. Allergy 28: 39-49 .

37. Sporik, R. B., M. D. Chapman, and T. A. E. Platts-Mills. 1992. House dust mite exposure as a cause of asthma [Editorial]. Clin. Exp. Allergy 22: 897-906 [Medline].

38. Platts-Mills, T. A. E., D. Vervloet, W. R. Thomas, R. C. Aalberse, M. D. Chapman, and (co-chairmen). 1997. Indoor allergens and asthma. Third International Workshop, Cuenca, Spain. J. Allergy Clin. Immunol. 100: S1-S24 .

39. Calhoun, W. J., E. C. Dick, L. B. Schwartz, and W. W. Busse. 1994. A common cold virus, rhinovirus 16, potentiates airway inflammation after segmental antigen bronchoprovocation in allergic subjects. J. Clin. Invest. 94: 2200-2208 .

40. Cockcroft, D. W., R. E. Ruffin, J. Dolovich, and F. E. Hargreave. 1977. Allergen-induced increase in non-allergic bronchial reactivity. Clin. Allergy 7: 503-513 [Medline].

41. Platts-Mills, T. A. E., E. R. Tovey, E. B. Mitchell, H. Moszoro, P. Nock, and S. R. Wilkins. 1982. Reduction of bronchial hyperreactivity during prolonged allergen avoidance. Lancet 2: 675-678 [Medline].

42. Platts-Mills, T. A. E., M. D. Chapman, and L. M. Wheatley. 1999. Control of house dust mite in managing asthma: conclusions of meta-analysis were wrong [Letter to Editor]. Br. Med. J. (In press)

43. Peak, J. K.. 1998. Can asthma be prevented? Evidence from epidemiological studies of children in Australia and New Zealand in the last decade. Clin. Exp. Allergy 28: 261-265 [Medline].

44. Platts-Mills, T. A. E., R. B. Sporik, M. D. Chapman, and P. W. Heymann. 1997. The role of domestic allergens. In D. J. Chadwick and G. Cardew, editors. Rising Trends in Allergy. John Wiley & Sons, Chichester, UK. 173-189.

45. Gortmacher, S. L., A. Must, A. M. Sobol, K. Peterson, G. A. Colditz, and W. H. Dietz. 1996. Television viewing as a cause of increasing obesity among children in the United States. Arch. Pediatr. Adolesc. Med. 150: 356-362 [Abstract].

46. U.S. Department of Education, National Centre for Education Statistics. 1996. Digest of Education Statistics. U.S. Government Printing Office, Washington, DC. Publication No. (ES)96-133:64.

47. Luder, E., T. A. Melink, and M. DiMaio. 1998. Association of being overweight with greater asthma symptoms in inner city black and Hispanic children. J. Pediatr. 132: 699-703 [Medline].

48. Skloot, G., S. Permutt, and A. Togias. 1995. Airway hyperresponsiveness in asthma. J. Clin. Invest. 96: 2393-2403 .

49. Fredberg, J. J., D. S. Inouye, S. M. Mijailovich, and J. P. Butler. 1999. Perturbed equilibrium of myosin binding in airway smooth muscle and its implications in bronchospasm. Am. J. Respir. Crit. Care Med. 159: 959-967 [Abstract/Free Full Text].

50. Hark, W. T., W. M. Thompson, T. McLaughlin, and T. A. E. Platts-Mills. 1998. The influence of behavior on resting sigh and respiratory rates in normal and asthmatic subjects. J. Allergy Clin. Immunol. 101: S183 .

51. Lang, D. M., and M. Polansky. 1994. Patterns of asthma mortality in Philadelphia from 1969 to 1991.  N. Engl. J. Med. 331: 1542-1546 [Abstract/Free Full Text].