Why Evolution Hasn't Gotten Rid of Allergies

Allergies were formally named over a hundred years ago and only seem to be getting worse for people year after year where in most industrialized countries allergies have increased a lot in past decades where food allergies in particular have risen dramatically where as much as seasonal allergies cause suffering dying from peanuts or other food allergies is even more serious creating a growing public health concern.

At the heart of allergic reactions are lymphocytes a type of white blood cell that help guard the body against foreign substances where when they encounter a particle or cell with surface marker molecules identifying it as a foreign invader they produce antibodies specifically engineered to fight that particular threat where there are five basic types of antibodies called immunoglobulins but the one responsible for allergic reactions is IgE where 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 a large amount of ragweed IgE antibodies are produced and attach to mast cells where these antibodies and mast cells then lie dormant until the allergen is encountered again where after the second exposure the antibodies created after the first exposure are at the ready binding to the allergen and alerting the immune system where unlike viral infections where subsequent encounters pose little danger with allergens the opposite is true where subsequent exposures are usually worse than the first.

After the first exposure to an allergen a large amount of allergen-specific IgE antibodies are produced and attach themselves to cells called mast cells where mast cells are a type of cell that contain the histamines that define the allergic response where when the immune system destroys the allergen it also destroys the mast cell which causes it to release its payload of histamines where this causes sneezing wheezing and sniffling that characterize allergic reactions.

Sometimes the allergic response can proceed to the point of danger where some people develop hives or begin having breathing difficulties where body tissues may swell like those in the throat where blood pressure drops and anaphylactic shock can happen in as little as three to four minutes after exposure to the allergen where if it’s not treated right away usually with epinephrine it can lead to death making anaphylaxis a medical emergency requiring immediate intervention.

Parasites are more common in this world and in our bodies than we likely realize where helminths like hookworms or tapeworms are large macro parasites many of which are soil transmitted and infect the digestional tract where more than 20 percent of people on Earth are dealing with some sort of parasite infection where humans have been dealing with these worms throughout our evolutionary history creating strong selection pressure for immune defenses against them.

The way our body reacts to parasitic worms is surprisingly similar to the way it reacts to allergens where 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 where before this was discovered scientists didn’t know any other purpose for the IgE antibody besides allergies revealing a potential evolutionary origin for the allergic response mechanism.

Scientists theorized that there must be some form of molecular similarity between the proteins that cause allergies and the proteins on the surface of parasitic worms where they found that molecules of plant allergens do indeed bear a striking resemblance to molecules produced by helminths where 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 bilharzia a tropical disease where using computer analysis researchers found over 2000 parasite proteins that show significant similarity to allergenic proteins.

One study showed that mice engineered to be unable to make IgE can still defend against parasitic worms and conversely individuals with high levels of IgE fared no better against the worms where this suggests that perhaps IgE is part of a worm defense system but not the main component where this begs the question what is IgE actually for if it’s not even that important in defense against helminths raising doubts about the simple molecular mimicry explanation for allergies.

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 where rather than viewing allergies as an immunological mistake some scientists propose they serve a protective function where all those sneezes coughs and itchy eyes might just be stopping something worse from getting in where this evolutionary perspective suggests that the short-term suffering of allergic responses prevents long-term harm from toxins and venoms creating net positive selection pressure.

When mast cells burst and histamines are released in the upper gastrointestinal tract immediate vomiting occurs in the lower gastrointestinal region diarrhea in the respiratory tract constriction of the throat and coughing in the nasal passages sneezing and in the skin itching that leads to scratching where all these responses are methods to expel something as if the body is in immediate danger where allergies seem like they could have evolved to be the last line of defense against toxins venom and poison where this toxin theory of allergies was first proposed in 1991.

Most allergens like hay dust peanuts wheat or shellfish are not toxic themselves but what is toxic are fungal spores that often invade hay and dust aflatoxins produced by parasitic fungi that often contaminate peanuts and grain and paralytic toxins produced by algae or plankton that often contaminate shellfish where 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 creating an adaptive benefit where allergen avoidance trains the host to avoid potential toxins.

The chemicals released by mast cells during anaphylactic shock cause dilation of the peripheral blood vessels leading to peripheral pooling of the blood that results in a drop in blood pressure where this response serves to keep the poison or potential poison away from vital organs where why this sometimes proceeds to deadly levels in some people and not others is still not known representing an ongoing mystery in allergy research.

In 2013 researchers used mice and bee venom to test the toxin theory where they first injected one group of mice with small amounts of bee venom equivalent to one or two stings and another with saline then three weeks later they injected both groups with a potentially lethal dose of bee venom where the mice that had been previously injected with small doses of venom fared significantly better being three times more likely to survive where they then tested three different kinds of genetically modified mice without IgE without IgE receptors and without mast cells altogether where these mutant mice did not benefit from initial exposure proving the allergic pathway is protective.

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 where environmental factors like air pollution chemical exposures processed foods and reduced microbial diversity may all contribute to immune dysregulation and increased allergic sensitization creating a multifactorial explanation for rising allergy rates in industrialized societies.

The hygiene hypothesis suggests that children are increasingly raised in a sterile environment getting fewer infections like the parasitic ones mentioned earlier where the thinking is with fewer parasites to fight the immune system turns against things that should be harmless where this hypothesis proposes that reduced microbial exposure during childhood development leads to immune system dysregulation making allergies more common in industrialized nations where cleanliness paradoxically increases allergic disease burden.