For most of us, springtime means sunshine, green grass, and blooming flowers.    But for some, it can also mean sneezing and watery eyes, or even trouble breathing.    We’re talking about allergies, and almost anything can cause them: grass, flowers, ragweed, peanuts, bee stings, penicillin, soy, latex, … the list goes on and on.    An estimated 40 percent of the world’s population suffers from allergies, and that number is on the rise.    But how can a peanut, so small and simple and delicious be so deadly?   And what even ARE allergies, anyway? How are they caused? Can they be prevented, or even cured?    Well, to understand allergies, we first need to talk about your immune system.   Your immune system is meant to keep you healthy, but in people with allergies, they tend to overreact.   And you can lay the blame on your lymphocytes, or white blood cells.   Lymphocytes are like little hall monitors, traveling around your body on the lookout for antigens -- foreign invaders like parasites, bacteria and viruses.    When a lymphocyte detects an antigen, it begins producing large, y-shaped proteins called antibodies.   Humans have almost ten billion different kinds of antibodies, and each one binds to a specific antigen, neutralizing the threat. It’s like having the keys to ten billion different locks.   But in an allergic person’s immune system, the lymphocytes get confused. They treat allergens like they’re antigens.    Allergens themselves are really just a kind of enzyme, called an antigenic protein.   Scientists don’t know what it is about the structure of these proteins that causes such alarm in some people's immune systems. They don’t resemble viruses or bacteria, but the immune system still treats them like a threat.    But doctors do know that while thousands of substances can be allergens, some are much more likely to send your immune system into overdrive.    That’s why just eight foods account for 90% of all food allergies ---- tree nuts, eggs, soy, peanuts, fish, shellfish, milk, and wheat.    When someone’s first exposed to an allergen, their lymphocytes create a bunch of antibodies called immunoglobulin E, or IgE.    Everybody has IgEs. And when they attach to the surface of certain immune cells, those cells then release enzymes that help fight infections.    It’s all part of a defense system that evolved to protect us from parasitic worms and bacteria, and it’s pretty effective. At least, when it’s working properly.   But when an allergic person’s lymphocytes are faced with an allergen, their immune cells freak out and overproduce these enzymes, causing the symptoms of an allergic reaction.    These can include a runny nose, itching, or hives -- localized swelling on the skin. More severe reactions can involve nausea, vomiting, or even trouble breathing.    The severity of these reactions is dictated by a wide variety of factors, like how much of an allergen is in the body, and how concentrated the immune cells are that have IgE’s bound to them, and how much of the enzymes they’re producing.    In some people, the histamine enzyme can be the problem.   Histamine dilates blood vessels and increases mucus production, allowing infection-fighting cells to travel to an affected area. Too much of it can cause itching or a runny nose.   But immune cells in other people might release a lot of an enzyme called tryptase, which is linked to the absolute worst reaction you can have… anaphylactic shock.     An overload of histamine and tryptase can cause your blood pressure to plummet. Then the bronchial tubes constrict, making it harder to breathe… and in some cases, the throat can swell, too, cutting off the oxygen supply completely.   Hundreds of people in the US die every year from anaphylactic shock. Which is why those who have severe allergies usually carry an epinephrine shot, just in case.    Epinephrine is a form of adrenaline. When it’s injected, it constricts the blood vessels and eases swelling, allowing the sufferer to, hopefully, breathe again.    The effects only last about twenty minutes, though, so the person will usually get themselves to a doctor.   So that’s how allergies work. But why do we have allergies in the first place?!   I mean, how does it make sense that something totally harmless to one person can be lethal to another?   Well, it doesn’t make any sense, at least not yet. Scientists can’t quite agree why we have allergies.    They do know that allergies have a genetic component.    Studies have shown that if you suffer from allergies, there’s a 33 percent chance that your kid will develop allergies as well. And if both parents have allergies, that risk jumps to 70 percent.    But it’s not that the child inherits specific allergies. They’re just more likely to develop them. So a parent who has an allergy to eggs could have a kid who isn’t allergic to eggs, but is allergic to peanuts.   Kids can outgrow their allergies, though. The extra IgEs that their bodies produce are at their highest levels before the age of 10, but they can drop drastically by the time they reach 30.    And if someone has an allergy to one antigenic protein, there’s a good chance they’re allergic to other proteins, as well. That’s called cross-reactivity.    So if you’re allergic to ragweed pollen, you’re more likely to be allergic to apples, because the protein structure in both are very similar. That’s why the immune system treats them as the same antigen.   Why certain people develop allergies isn’t the only thing puzzling scientists, though.   Allergies around the world have been rising since the early 1980s. Reports of food allergies among children in the US, for example, increased by 20 percent from 1997 to 2007.     Some scientists suggest that air pollution might be to blame.    Three studies in the early 1990s linked pollutants like sulfur dioxide and carbon monoxide to allergic asthma, which can cause shortness of breath and wheezing.    In one series of studies, researchers found lower instances of allergies among children in what was then East Germany, compared to those who grew up in the more developed West Germany.    Even though the kids in East Germany were exposed to a lot more air pollution than those living in the West, scientists concluded that the Western pollution contained more carbon monoxide from automobile exhaust.    But there are other theories, too -- like the idea first developed in 1989 known as microbial deprivation.   This model essentially suggested that the modern, industrialized world is just too dang clean.    We know that allergies are highest among those who live in wealthy, developed countries and in urban areas, regardless of race or sex.    According to microbial deprivation, children who grow up in these environments simply aren’t subjected to enough dirt, grime, bacteria, and parasites… depriving them of what scientists call benign exposure.   The hypothesis suggests that this benign exposure helps balance the immune system, giving it something to fight. Maybe when there aren’t enough pathogens to keep it busy, it overreacts and attacks harmless things like pollen, instead.    There is some research to back this up, too.    In 1998, one study suggested that children who went to daycare, and thus were exposed to lots of viruses, had lower instances of allergies than those who didn’t go to daycare.   And in 1999, a Swedish scientist noted that farmers’ children who had regular contact with animals before the age of 7 and drank unpasteurized milk, didn’t develop as many allergies as children who lived in the city.   Now, these scientists aren’t suggesting we return to the days of dirty drinking water, because a lot of those parasites and bacteria, like dysentery, killed people.    200 years of sanitation have brought a lot of good to the world.   But researchers are noticing a correlation between an environment devoid of microbes, and an increase in allergies.    This link has led to an experimental form of treatment called Helminth Therapy.    Helminths are parasitic worms, like hookworms, that people deliberately infest themselves with in order to ease their allergies. Sounds kinda gross, right?   But recent studies are showing that it might actually work.    For instance, a study in 2012 found that patients infected with worms needed less medication that those who were worm-free. The down-side is, those worms also gave the patients stomach aches, bad gas, and diarrhea.    Yeah… I mean, parasitic worms will do that to you.   Thankfully, there are ways around allergies that are slightly less horrifying.   For example, in many cases, we actually know which antigenic proteins are causing the reaction, so we can get around them.   Like when it comes to eggs, we know they have four different antigenic proteins in the whites alone, plus three more in the yolk.    Meaning that someone whose immune system reacts to the antigenic protein in the white could be perfectly fine eating the yolk.    And, some of these proteins, specifically those in eggs and milk, can be destroyed by extensive and prolonged heat.    One study showed that only 4 percent of children with an egg allergy reacted to a muffin after it was cooked for 30 minutes at 175 degrees Celsius. Heating the proteins had changed them enough that they weren’t triggers anymore.    And then there are the more traditional forms of allergy treatments, like immunotherapy, which works by changing the way your body responds to allergens.    Once or twice a week, over the course of a year, a patient is injected with tiny doses of an allergen. Because the exposure is so low and so constant, the immune system gradually stops responding with IgE antibodies.    Instead, it begins responding with IgG antibodies, another, more common type of antibody that fights chronic infections.    These IgGs don’t bind with immune cells first, so they don’t release histamines or other chemicals that cause an allergic reaction.   Instead, they bind to an antigen and neutralize it directly.   The downside of these shots is that they’re really expensive, time-consuming, and don’t work on severe allergies, which can be triggered by even those small doses.   To treat more severe allergies, researchers are looking into anti-IgE immunotherapy, which directly blocks the production of IgE and just takes the whole thing out of the equation.    In this kind of treatment, patients are given shots of a drug called omalizumab, which binds to existing IgEs in the blood, rendering them useless. It also blocks future production of the antibody by binding to the lymphocytes that make it.    But then guess what! Some people -- although really, very few -- are allergic to omalizumab!    So the safest option, at least for people with food allergies? An avoidance diet, which is the official name for what the rest of us would call common sense: just don't eat what you’re allergic to.   So if you’re allergic to shellfish, skip the lobster. Boom. Problem solved.   Thanks for watching this episode of SciShow, which was made possible by our latest President of Space Conner Tart, who was nominated for that position by Michie, Orin, Julia and Jack.They say "Congratulations Conner! We are so, so proud of you and excited for you to start your next great adventure. We love you!"   If you want to help us keep making videos like this, check out patreon.com/scishow. And don’t forget to go to youtube.com/scishow and subscribe!