How do allergies work?

It’s a bright, sunny spring day. The trees are greening up, and the flowers are starting to bloom. All of a sudden, you start sneezing like crazy, and your eyes look like you’ve been crying for days. Or, maybe your cat likes to hang out on your bed at night and your nose always seems stuffy while you’re trying to fall asleep (thanks a lot, Beeker!). While allergies aren’t too much fun to deal with, the science behind allergic reactions is amazing!

What are allergic reactions?

The heart of the problem is actually a case of mistaken identity. Your immune system normally recognizes and destroys pathogens (viruses, bacteria, fungi, etc.) that make their way into your body, which is a good thing: your body is constantly under attack by foreign invaders. But in an allergic reaction, the immune system acts like a bully and goes around picking fights with innocent bystanders. Instead of fighting harmful pathogens, the immune system starts recognizing harmless things like pollen and animal dander as dangerous and mounts full-blown immune reactions whenever it comes across these harmless things.

This can even be taken to an unfortunate extreme where your body recognizes some of its own cells as a foreign invader and kills them. This is what happens in autoimmune diseases like lupus or rheumatoid arthritis.

How do people become allergic to things?

Allergies tend to run in families, but no one is ever automatically allergic to something. There are billions of potential allergens (proteins and other molecules that cause allergic reactions) in the world—how would your body know it was allergic to something if it had never encountered it before? Instead, your body first needs to interact with something new, recognize it as an invader, and take steps to defend against it if it should ever come back. This process is called sensitization.

Some of the cells floating around your bloodstream are B cells, a type of white blood cell. They’re sort of like mall cops: they patrol the body, check things out, and make sure everything is going swimmingly. If they find something new and “dangerous” in the form of an allergen, they will actually swallow it in a process called endocytosis, and take their newly-won prize back to a lymph node.

Inside the lymph node, the B cell becomes activated by a T cell (another type of white blood cell), and it morphs into a new type of cell: a plasma cell. The plasma cell’s main job is to now produce IgE antibodies specific to that one type of protein, and it does this very well: they can crank out thousands of antibodies every second (pew pew!).

The IgE antibodies float around in the bloodstream like microscopic “Wanted” posters, and are picked up by two other types of white blood cells: basophils, which circulate around your body in your bloodstream, and mast cells, which live fixed in place in areas that are in close contact with the outside world, like your skin, mucus membranes, and intestines. The basophils and the mast cells attach the IgE antibodies to their cell membranes in a process called priming, and if the same allergen happens to come along again, they can now recognize the threat (sort of like seeing Billy the Kid come along after seeing his “Wanted” poster).

The process of sensitization is now complete: the person is now primed and ready to have an allergic reaction if they encounter the same protein again.

What happens in an allergic reaction?

Now that the mast cells and the basophils are fully primed and ready to go, all they need is to encounter the allergen again before unleashing a storm of chemicals that actually cause the symptoms of an allergic reaction.

Basophils and mast cells are both granulated, meaning that if you look at stained cells under a microscope you will see tiny, dark granules inside of them. These are actually tiny packets of chemicals like histamine, leukotrienes, and cytokines, among others. When the IgE antibodies on the surface of the basophils and the mast cells actually come into contact and bind with the specific type of allergen, it triggers the cell to degranulate—to rapidly dump the contents of the tiny chemical packets out of the cell, where they can act in the body.

This chemical cocktail creates a whole new chain of reactions that ultimately ends up causing the familiar symptoms of allergies—everything from itchy, watery eyes; stuffy noses; hives; etc. In extreme cases, it can cause the throat to dilate and cause an asthma attack, or even cause so much swelling over the body that a person could go into anaphylactic shock and die.

This is why some people need to carry EpiPens—the reaction happens so fast that there’s no time to call 911 and get to a doctor. The person could literally die before that happens. EpiPens deliver an instant dose of epinephrine, the only chemical that acts fast enough to reverse the effects of the allergy chemical cocktail fast enough to save a person’s life.

It’s a bit scary to think that right now, at this moment, your body contains huge numbers of cells with tiny chemical packets inside of them that could literally kill you if enough of them were to activate—or, they might just make you sneezy, sleepy, dopey, or grumpy (but not happy or bashful!). Scientists still have a lot to learn about how allergies work, but in the meantime, they still are a fascinating thing to think about!

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Written by Lindsay VanSomeren

Lindsay graduated with a master’s degree in wildlife biology and conservation from the University of Alaska Fairbanks. She also spent her time in Alaska racing sled dogs, and studying caribou and how well they are able to digest nutrients from their foods. Now, she enjoys sampling fine craft beers in Fort Collins, Colorado, knitting, and helping to inspire people to learn more about wildlife, nature, and science in general.

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