Ah, glorious springtime. It brings flowers, warmer temperatures—and for many, incessant sneezes and sniffles. Everybody curses allergies as annoying at best, and some allergic reactions—such as anaphylaxis, which rapidly lowers blood pressure and closes the airways—can be fatal. But a handful of researchers now propose that allergies may actually have evolved to protect us. Runny noses, coughs and itchy rashes keep toxic chemicals out of our bodies, they argue, and persuade us to steer clear of dangerous environments.

Most immunologists consider allergies to be misdirected immune reactions to innocuous substances such as pollen or peanuts. Viral and bacterial infections invoke what are called "type 1" immune responses, whereas allergies involve "type 2" responses, which are thought to have evolved to protect against large parasites. Type 1 responses directly kill the pathogens and the human cells they infect; type 2 works by strengthening the body's protective barriers and promoting pest expulsion. The idea is that smaller pathogens can be offensively attacked and killed, but it's smarter to fight larger ones defensively.

But Ruslan Medzhitov, an immunobiologist at Yale University, has never accepted the idea of allergies as rogue soldiers from the body's parasite-fighting army. Parasites and the substances that trigger allergies, called allergens, "share nothing in common," he says—first, there are an almost unlimited number of allergens. Second, allergic responses can be extremely fast—on the scale of seconds—and "a response to parasites doesn't have to be that fast," he says.

In a paper published April 26 in Nature, Medzhitov and his colleagues argue that allergies are triggered by potentially dangerous substances in the environment or food to protect us. (Scientific American is part of Nature Publishing Group) As evidence, they cite research including a 2006 study published in The Journal of Clinical Investigation reporting that key cells involved in allergic responses degrade and detoxify snake and bee venom. A 2010 study published in the same journal suggests that allergic responses to tick saliva prevent the pests from attaching and feeding. This mechanism, he argues, is distinct from the classic type 2 response the body uses to defend itself against internal parasites.

More generally, hated allergic symptoms keep unhealthy environmental irritants out of the body, Medzhitov posits. "How do you defend against something you inhale that you don't want? You make mucus. You make a runny nose, you sneeze, you cough, and so forth. Or if it's on your skin, by inducing itching, you avoid it or you try to remove it by scratching it," he explains. Likewise, if you've ingested something allergenic, your body might react with vomiting. Finally, if a particular place or circumstance ramps up your allergies, you're likely to avoid it in the future. "The thing about allergies is that as soon as you stop exposure to an allergen, all the symptoms are gone," he says.

Importantly, Medzhitov notes that although allergies are intended to be helpful, they are sometimes excessive and detrimental—the body can go too far. And allergies don't always make sense. "I would say that food is still mostly innocuous," says Dale Umetsu, an immunologist at Children's Hospital Boston, yet "food allergies affect one in 12 kids." How is that protective? According to Medzhitov, foods may have proteins in them that are harmful or they might mimic potentially harmful substances. (With food, he says, there's often little consensus about what, exactly, the offending allergen is.) And one has to think of the evolutionary past, he adds: for our ancestors hundreds of thousands of years ago, many plants that looked like food were toxic, so allergies may have evolved to protect us from them. Finally, he says that some allergies may develop through a "guilt by association" mechanism: An individual might develop an egg allergy after eating eggs in a polluted environment, for instance.  "This is a type of detection by proxy—you use some cue, like smell, or a visual cue or taste, to indicate if a food is associated with something that's noxious. Next time you're exposed to it, you avoid it."

10 Comments

1. 1. not-evolved 04:34 PM 4/25/12

   The article states: "allergies involve 'type 2' responses, which are thought to have evolved to protect against large parasites" but gives no indications why it is thought that we did not have them from the beginning.
   
   Why does conjecture need to be added. Suggesting that this response “evolved” did not add anything to the understanding of the ideas put forth in the article.

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2. 2. faucets 06:20 PM 4/25/12

   "Large" is a matter of scale. The apparent function of IgE, the immune component most often implicated in allergies, reacts to potential attacks by agents that can't be engulfed by a macrophage. That ain't so big.

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3. 3. 13inches 08:06 PM 4/27/12

   not-evolved:
   
   You seem to be trolling the S.A. forums looking for the key words 'evolved' or 'evolution', and when you find these words you make a silly post that makes no sense. The S.A. boards are devoted to the discussions of SCIENCE, not mythology. Go back to your Flat Earth Society meetings and your Creationism meetings and stop wasting our time.

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4. 4. wakeele 08:59 PM 4/28/12

   As a subscriber and online reader, and as a student of science, I would like to see references cited, and for papers and books mentioned in your articles (particularly when they are incompletely mentioned in passing), if used. The three papers mentioned in the fourth paragraph is a great example. There is enough information to get one started if they want to try to locate these papers, but it would certainly would promote further research on the topic if they had clearcut references to go directly to--particularly with readers that don't just settle with what they're told from news sources. It took me about 10-15 minutes to find Medzhitov's (et al.) recent paper after using a few different techniques. I probably could have spent a half hour if I wanted to locate them all to read later.
   
   I know SA isn't a scholarly journal, nor am I looking for it to be as thorough and void of colorful writing, but a few references at the end of articles would be a nice touch. There are the further reading suggestions at times, but don't often contain some of the works referenced in the article itself. In an age of settling on one generalized news source, we sure could use promotion of visually noticeable references for the content. Thanks.

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5. 5. collettedesmaris 05:52 AM 5/1/12

   Due to the limitation of characters that this forum imposes, it dictates that I must submit my Comment in several posts. Commencing with Part 1:
   
   Part 1.
   An allergic reaction is an overactive immune response to an otherwise non-pathogenic (doesn't normally cause illness)stimulus - not a"misdirected immune reaction", as you stated. It is an"overactive"or"hypersensitive" response to something(called an allergen)to which an individual is allergic or hypersensitive.The allergen can be food, medication, animal dander, an insect
   sting, the waste matter from dust mites; etc.
   
   The hypersensitivity reaction that most people are familiar with, and refer to as having "allergies",is mediated by a substance called "Histamine"(hence, the medicine for treatment of some allergies called an "Anti-histamine".)
   
   Your statements that "...allergies may actually have evolved to protect us"; followed by:"Runny noses, coughs and itchy rashes keep toxic chemicals out of our bodies ..." are completely erroneous. The runny nose and rash that are symptoms of an individual's allergic reaction to something, are not present
   to serve the purpose of "keeping toxic chemicals out of our bodies". Nor has an individual's allergic reaction evolved to protect them.Conversely, it is
   literally the subsequent exposure after the first one, that can be far worse to one who is acutely allergic.
   
   You demonstrate complete inaccuracy in the paragraph that begins with the sentence:"Most immunologists consider allergies to be misdirected immune reactions to innocuous substances .... " Your premise that "viral & bacterial infections invoke type 1 immune responses, and allergies involve type 2 responses that have evolved to protect against large parasites"; is completely invalid.I've got news for you - there are not just two "types" of reactions - there are currently four - with some in our field of endeavor favouring a classification system of seven. Regardless of whether there are two, four, or seven - the classification of "Type I" reactions have absolutely nothing to do with "viral & bacterial infections invoking" them - the fact that you would make such a blatant mistake regarding data of this nature is appalling. I am beside myself in an attempt to assimilate any sort of understanding as to how an error of this magnitude could possibly have been made in a publication that calls itself "Scientific American."
   
   You also stated in your article that "allergies are helpful".
   
   (Continued - Part 2.)
   
   
   
   
   

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6. 6. collettedesmaris 05:56 AM 5/1/12

   Part 2.
   
   It escapes me where you got the notion that any allergy has the propensity to be "helpful" - that statement is ridiculously invalid.
   
   Since the content, as well as the premise for your entire article is glaringly incorrect, let me clarify some things for you regarding allergies and allergic reactions:
   
   (I.)Type I reactions(ie, Immediate Hypersensitivity Reactions)involve Immunoglobulin E(IgE)–mediated release of histamine and other mediators from Mast Cells and Basophils.
   (II.)Type II reactions(ie, Cytotoxic Hypersensitivity Reactions)involve Immunoglobulin G or Immunoglobulin M antibodies bound to cell surface antigens, with subsequent complement fixation.
   (III.)Type III reactions(ie, Immune-Complex Reactions) involve circulating antigen-antibody immune complexes that deposit in postcapillary venules, with subsequent complement fixation.
   (IV.)Type IV reactions(ie, Delayed Hypersensitivity Reactions,Cell-Mediated Immunity) are mediated by T cells rather than by antibodies.
   
   Some in our field believe the classification system may be too general,and favour a more recent system of
   classification proposed by Sell.[1] This system accounts for the fact that multiple components of the
   immune system can be involved in various types of hypersensitivity reactions; and it divides Immuno-
   pathologic responses into the following categories:
   
   Inactivation/Activation Antibody reactions
   Cytotoxic or Cytolytic Antibody reactions
   Immune-Complex reactions
   Allergic reactions
   T-cell Cytotoxic reactions
   Delayed Hypersensitivity reactions
   Granulomatous reactions
   
   [1] Sell,S.; Rich,R.R.; Fleisher,T.A.; et al,eds. "Clinical Immunology:Principles and Practice". ed.St. Louis, Mo: Mosby-Year Book
   "Allergic reactions manifest clinically as Anaphylaxis, Allergic Asthma, Urticaria, Angioedema, Allergic Rhinitis, some types of drug reactions,and Atopic Dermatitis.These reactions tend to be mediated by IgE,which differentiates them from Anaphylactoid reactions that involve IgE-independent Mast Cell and Basophil degranulation." [2]
   [2]Lawlor,G.J.;Fischer,T.J.;Adelman,D.C. - eds. "Manual of Allergy and Immunology"3rd ed Phila.,PA: Lippincott-Raven
   
   "Patients prone to IgE-mediated allergic reactions are said to be 'Atopic'. Atopy is the genetic pre- disposition to make IgE antibodies in response to allergen exposure."[3]
   [3] Nimmagadda S.R.;Evans, R. 3rd."Allergy: Etiology and Epidemiology."
   
   (Continued - Part 3)
   
   
   
   
   
   
   

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7. 7. collettedesmaris 05:59 AM 5/1/12

   Part 3.
   
   Now then, Ms. Moyer - it is abundantly clear that you are not in compliance with the principles and standards of practice of the Society of Professional Journalists; as professional integrity is the corner-
   stone of a Journalist's credibility. Journalists should seek the truth and report it - it is the duty of their profession to test the accuracy of information from all sources; and exercise great care to avoid inadvertent error. Deliberate distortion is NEVER permissible. We are addressing this with you, because this article is so glaringly and ridiculously invalid, that you could not possibly have performed even a modicum of research in obtaining the information. This article unarguably gives rise to the question:
   "Just what kind of show are you people running over there now?"
   
   Against our better judgement, we'll give you the benefit of the doubt; and presume that you will certainly be accountable for what you've laid down here; and respond by explaining that you inadvertently got the information in this article mixed up with the information for another article .... and it'd be best if you just left it at that. I mean, really - what else could you say? That you're in Elementary School?
   
   In closing - for you to say nothing; is saying
   something.

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8. 8. collettedesmaris in reply to 13inches 06:30 AM 5/1/12

   13 Inches - in all fairness, 'not-evolved' stated a valid inquiry - that you would make mockery of the inquiry is extremely inappropriate. As well, the liberty you take by informing him/her what the Scientific American forums are for; combined with the instruction for him/her to "go away", as it were; insinuates that you think you have the authority to do so. I am of the educated opinion that the comment that
   'not-involved' made was not at all silly, and it made complete sense to me. Kindly explain what gives you the right to believe it is acceptable to speak to anyone in such a derisive manner; and just who the he__ do you think you are to speak to anyone in such an authoritative manner; as though you have the right
   to, or are in the position to do so?
   
   I found your personal attack of "not-evolved" extremely offensive as well as abusive. I'm here to tell you, tough guy; it really gives pause for one to wonder what motivated you to submit such a post.
   
   Of paramount importance, for you to make the statement that "the S.A. boards are devoted to the discussions of science .." under the umbrella of this ridiculously inaccurate article, is ludicrous; if not outright laughable. What were you thinking?!
   
   

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9. 9. bucketofsquid in reply to collettedesmaris 09:59 AM 5/3/12

   Thanks for the update on terminology. Back in the days when I was trying to understand my allergies they always lumped T-cells and B-cells under the label antibodies. It is nice to know that we are more specific now.
   
   I notice that you dislike the way this article presented things but did not directly address the hypothesis presented by Ruslan Medzhitov. Have you read his work and do you have an opinion on it? Is he simply out of date on terminology or was his work taken out of context?

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10. 10. bucketofsquid in reply to collettedesmaris 10:14 AM 5/3/12

    In regards to the posts by Not-evolved and 13Inches; they are both forum trolls. Not-evolved always seems to take exception to anything that challenges a "magical creation" theory of existence. I have yet to find anything even vaguely resembling evidence that the Earth and all of the universe is only a few thousand years old. There are many such as myself that firmly believe that God created the entire universe via a process that obeys physical laws and required billions of years to reach our current state of existence. For that reason the challenge of the use of the term evolved should have a basis in science rather than superstitious shamanism.
    
    We are no longer in the era of burning witches and one would expect somewhat more intelligent debate on the topic posted on a forum where that topic is under discussion. This topic is allergies and not the magical branch of creationism.
    
    As for 13Inches; Most of their posts are noxious but occasionally there is a post that is brilliant. Obviously this is a person that has never had to mature. I hold out hope for this person because many of my earlier posts were little better. As I've gotten older I have moderated my behavior and hopefully am not so abrasive and intolerant. I guess I'm evolving as a person.

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