Why Evolution Hasn't Gotten Rid of Allergies

after a long winter the sun is finally out the flowers are blooming and it's time to take in the beautiful weather your immune system is delighted it begins celebration mode it helps you mark the occasion with itchy eyeballs chapped nose sneezing and my favorite almost but not quite sneezing it definitely feels like your immune system is trying to kill you but it insists that this is what needs to happen you shut your windows stock up on claridin and accept this fate because you have no other choice allergies were formally named over a hundred years ago and only seem to be getting worse for people year after year in most industrialized countries allergies have increased a lot in past decades food allergies in particular and as much as seasonal allergies suck dying from peanuts sucks even more why on earth do our bodies freak out about things that should otherwise be harmless evolution usually provides an explanation for our bodies doing things even if those things are detrimental to us in the current day and age sickle cell disease for example can be deadly but the irregular shaped red blood cells that it's defined by has historically given a level of protection against malaria the world is plagued by an obesity epidemic because our caveman ancestors needed all the extra energy they could get making us store extra calories as fat and making us want to eat even when we don't actually need to but sneezing for a month straight every spring or going into anaphylactic shock because you touched a peanut what help has that ever given us as a species pollen isn't toxic peanuts are not inherently poisonous it's our body's response to them that's the problem conditions like this that cause swift death especially as a child tend to be weeded out of the gene pool so why are allergies still here are allergic reactions just an evolutionary glitch or is there some secret to why we need them at the heart of allergic reactions are lymphocytes a type of white blood cell they help guard your body against foreign substances when they encounter a particle or cell with surface marker molecules that identify it as a foreign invader they produce antibodies specifically engineered to fight that particular threat there are five basic types of antibodies called immunoglobins but the one responsible for allergic reactions is ige each ige antibody can be very specific binding to one type of pollen for example but not another the first time an allergy prone person encounters an allergen like ragweed pollen for example a large amount of ragweed ige antibodies are produced and attach themselves to cells called mast cells mast cells are a type of cell that contain the histamines that define the allergic response these antibodies and mast cells then lie dormant until the allergen is encountered again then after the second exposure to an allergen the antibodies that were created after the first exposure are at the ready they bind to it alerting the immune system that it needs to be destroyed this is similar to how the immune system destroys viruses but unlike in a typical immune response against a virus in an allergy response the antibodies are attached to mast cells when the immune system destroys the allergen it also destroys the mast cell which causes it to release its payload of histamines this is what causes a person to suffer from an allergic response and causes the sneezing wheezing and sniffling most of us know all too well sometimes this allergic response can proceed to the point of danger some people develop hives or begin having breathing difficulties body tissues may swell like those in the throat blood pressure drops anaphylactic shock can happen in as little as three to four minutes after exposure to the allergen and if it's not treated right away usually with epinephrine it can lead to death with a viral or bacterial pathogen subsequent encounters after the first usually pose little danger since the body has built up a defense against it but with allergens the opposite is true subsequent exposures are usually worse than the first all this explains the what and the how but the why is less clear it's hard to see why natural selection would have produced allergies it feels like this is an immune response designed just to make us miserable but like all things with evolution there is likely a purpose or at least an explanation and one idea involves something unexpected something wriggling in the ground or even in your body parasites are more common in this world and in our bodies than we likely realize helmets like hookworms or tapeworms are large macro parasites many of which are soil transmitted and infect the digestional tract more than 20 percent of people on earth are dealing with some sort of parasite infection we as humans have been dealing with them throughout our evolutionary history and the way our body reacts to these worms is surprisingly similar to the way it reacts to allergens when a worm invades the immune system recognizes proteins on the surface of them and responds with not just any antibodies but ige antibodies the same antibodies that cause allergic reactions before this was discovered scientists didn't know any other purpose for the ige antibody besides allergies but what do pollen and worms have to do with each other scientists figured that there must be some form of molecular similarity between the proteins that cause allergies and the proteins on the surface of these worms a few years ago they put this theory to the test they found that molecules of plant allergens do indeed bear a striking resemblance to molecules produced by helminths for example an allergen found in birch pollen that is responsible for many allergic symptoms in early spring mimics a protein found in the parasitic worm that causes bilarzia a tropical disease caused by parasitic worms in fact using computer analysis researchers found over 2 000 parasite proteins that show significant similarity to allergenic proteins so by this theory allergies are just a case of mistaken identity an unfortunate side effect of defense against parasitic worms this has been a prevailing theory since the 80s and many studies support it and maybe this is part of the story but it can't be the full story while it's true that the immune system makes ige when it detects parasitic worms one study showed that mice engineered to be unable to make ige can still defend against the worms and conversely individuals with high levels of ige fared no better against the worms so perhaps ige is part of a worm defense system but not the main component so this begs the question what is ige actually for if it's not even that important in defense against helmets the ige antibodies surely evolved for a different bigger reason and on top of that not all allergens mimic the parasite proteins some plant ones do yes but other common allergens like nickel or penicillin have no resemblance it's unlikely then that the ige antibody evolved specifically to defend against helmets alone perhaps the shape of the allergens and what they resemble isn't the important thing but rather what they may be associated with the way allergic responses have been viewed all along is that they're an immunological mistake but some scientists don't buy it the persistence of allergies through time despite its cost and burden seems to strongly imply that there's a reason for them some adaptive benefit they give us to give us a clue let's look at what happens when the allergic cascade is triggered when mast cells burst and histamines are released in the upper gastrointestinal tract immediate vomiting occurs in the lower gastrointestinal region diarrhea and the respiratory tract constriction of the throat and coughing in the nasal passages sneezing in the skin itching that leads to scratching what do these have in common they are all methods to expel something as if the body is in immediate danger but we know it's not response against viruses bacteria or protozoa what else would the body need to defend against throughout evolution what else would merit an immediate rapid response the body expelling it at almost any cost basically what could instantly harm you toxins venom poison if toxins bind to certain cells they can cause irreparable harm to neurological cardiovascular metabolic and a host of other systems coming across them requires urgent action since many toxins can be immediately harmful thus it would make sense for our bodies to have developed defenses against them allergies seem like they could have evolved to be the last line of defense but this conundrum remains besides certain venoms that contain allergens most allergens are not toxic themselves hay dust peanuts wheat or shellfish should be harmless but here's what is toxic fungal spores that often invade hay dust aflatoxins produced by parasitic fungi that often contaminate peanuts and grain paralytic toxins produced by algae or plankton that often contaminate shellfish and of course venom itself if a toxin is commonly coupled to a harmless protein in the natural environment then the immune system may have evolved to target the carrier proteins seeing them as guilty by association so if you eat a clam a peanut or in my case lobster something in your genetic past or your own past tells your immune system that what you just ate is risky stuff rather than take any chances it works very hard to clear it all out and a secondary adaptive benefit to this response is that it trains its host to avoid these allergens and potential toxins things it considers too risky to deal with i know i certainly have no desire to eat a lobster anytime soon but what good is anaphylactic shock surely that isn't a helpful response well it kind of is the chemicals released by mast cells cause dilation of the peripheral blood vessels leading to peripheral pooling of the blood that results in a drop in blood pressure to keep the poison or potential poison away from vital organs why sometimes it proceeds to deadly levels in some people and not others is still not known the toxin theory of allergies was first proposed in 1991 and for over a decade it sounded good but little scientific evidence existed to support it if the toxin theory of allergies was true then scientists wanted to see with data that allergies somehow do work to protect against toxins and in 2013 it finally got the evidence it needed using some mice and some bees bee stings are known for their allergic potential alongside the damaging venom so researchers wanted to see if the allergic response produced by a bee sting was somehow beneficial to protect against the venom they first injected one group of mice with small amounts of b venom equivalent to one or two stings and another with saline aka nothing then three weeks later they injected both groups with a potentially lethal dose of b venom and watched to see what happened to the two groups the mice that had been previously injected with small doses of venom fared significantly better they were three times more likely to survive but to really see if it was indeed the allergic pathway that made this difference the researchers ran the same test on three different kinds of genetically modified mice mice without ige mice without ige receptors on their mast cells and mice without mast cells altogether and what they found was clear that these three mutant mice types without the key components of allergic reactions did not benefit from the initial exposure to the venom this shows us why there would be a positive evolutionary pressure to keep the ige antibodies and mast cells and it totally goes against how we generally view allergies if allergies are always seen as detrimental something to be blocked with allergy shots and antihistamines we may be preventing a vital system from working properly in our body rather than focusing on how to completely block allergic defenses across the board researchers are now hoping to understand why some people's allergies climb to deadly levels or why allergies are on the rise one hypothesis suggests that our modern world is to blame for an increase in allergies with more pollution dietary changes and less exposure to microbes changing how our immune systems respond another the hygiene hypothesis suggests that children are increasingly raised in a sterile environment getting fewer infections like the parasitic ones we mentioned earlier the thinking is with fewer parasites to fight the immune system turns against things that should be harmless while these theories are some of the most compelling to date scientists still have many questions in an ongoing debate about why we have allergies it's going to take generations of smart people to unravel this mystery of the human body but in the meantime perhaps we can start to see allergies as more than an annoying glitch all those sneezes coughs and itchy eyes might just be stopping something worse from getting in making videos like this is incredibly rewarding for me i get curious about something and then get to spend my days finding out the answer and as i'm learning i get to share that with the world in a way that makes sense to me all while keeping myself employed however the world of youtube is not always sunshine and rainbows as any of you who have ever ventured into the comment section will understand on stressful days this kind of negativity can get to you it's easy to feel like what you're doing is never enough and it's hard to carve out time to take care of yourself all while juggling a demanding work and regular life schedule and this is why i've taken it upon myself to really get serious about taking care of my own mental physical and emotional health to keep me happy in my job and also to work to not let the negative aspects of it leak into my personal life and to help me do this i'm taking the advice of jonathan van ness in his ultimate self-care playbook on skillshare in it he stresses the importance of things like writing down reminders or affirmations of who you are what you're doing or what's important to you or keeping a self-care schedule for yourself so you can commit to activities for yourself just like you would with anything else for jvn that may be needlepointing for me it's gardening reading in the pool or running generally doing anything to get me off my computer for a while he also talks about the importance of letting go of failures something that i am so seriously bad at in work and in life we may fall short of our expectations of ourselves and in his lessons he reminds you how to get back on track without beating yourself up about it overall it may sound like obvious stuff but for me it's not it was a really light-hearted funny inspirational way to spend an afternoon and has got me excited to let go breathe and focus on the goals i have for myself this is just one of many courses on skillshare that can help you build life skills skillshare is an online learning community with thousands of inspiring classes for creative and curious people it's curated specifically for learning meaning there are no distracting ads and it's less than 10 dollars a month with an annual subscription but the first 1 000 people to sign up with the link in the description will get a free trial of skillshare premium so you can dive into classes about self-care scrapbooking or productivity there's no risk in trying it out and seeing how it can help you shape your life as always thanks for watching and if you're looking for something else to watch right now you can watch real engineering's latest video about the uncertain future of jet fuel and the end of cheap travel or my previous video the insane biology of the platypus you

Channel: Real Science

Views: 357,561

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Id: JG-VUOM03jU

Length: 17min 27sec (1047 seconds)

Published: Sat Jul 10 2021

Reddit Comments

[u/_justpassingby_]

Great video, and I appreciate the path to the current understanding, it was a nice mystery ride. Having 0 knowledge of allergies I was convinced by the parasite argument 100%. I suppose it's lucky science has higher standards than "sounds legit".

I think it's worth mentioning that although evolutionary reasons can be a good paradigm, there's nothing in the law of nature that says a current gene needs to come with net benefits. It could well be entirely harmful but recent or only affect the creature after breeding age, or any other of the numerous loopholes that escape a shallow treatment of the logic. I need to reread Richard Dawkins' the Selfish Gene, but I remember being blown away by how counter-intuitive such a damned simple idea can be in practice given a healthy playground a few billion years.