Explainer: What are allergies?

Leaves of three, let it be.
Berries white, take flight.

That old rhyme helps gardeners and hikers identify poison ivy. It’s good information to have, because at least three in every four people are allergic to poison ivy. They’ll get a maddening, itchy rash if they so much as brush up against this three-leaved, white-berried plant.

But some people are allergic to things that are harder to avoid than poison ivy.

Many things can cause an allergic reaction. Pollen is famous for it. Pet dander, insect bites and stings — even dust mites cause their share of allergies. Food allergies also are common. For some people, even a tiny bit of peanut or shellfish can cause a potentially life-threatening reaction. Other people are allergic to certain kinds of medicines. Allergies are even behind some types of asthma, a breath-stealing condition.

Big misunderstandings

So what causes allergies? Allergens. An allergen is a substance that triggers the immune system to inappropriately go into overdrive. Components of pollen, peanuts — all sorts of things — can be allergens. But why do allergens make people snort and sneeze, break out in rashes or have any of the other unpleasant reactions that allergies cause?

Really, it’s just a misunderstanding.

It’s not the allergens themselves that cause the problem. It’s the body’s immune system. For most people peanuts are a nutritious food. But when people are allergic to something, their immune systems react inappropriately to what should be harmless substances. Such as peanuts.

The immune system’s job is to recognize any dangerous substance that gets into the body — then get rid of it. For example, it fights off the germs that cause infections, such as colds and flu. It does that by recognizing the germs that cause them as alien substances. Then it sends targeted immune cells to kill or otherwise eliminate them. But sometimes the immune system works too well. It can be like a smoke alarm that blares every time you cook a pizza or light a candle. A bit of pollen or a single bee sting tends not to be that dangerous for most people. But for people whose bodies see those as allergens, the immune system goes on red alert. It starts fighting that minor disturbance as if it were a major threat.

Fortunately, most of the time allergies are just annoying. But not always. They can be deadly. Anaphylaxis (An-uh-fih-LAX-is) is an extreme allergic reaction. When it develops, several parts of the body react at the same time, sometimes leading to death. When the immune system gets confused about what’s dangerous and what’s not, it can cause trouble for the entire body.

Antibodies with time on their hands

The first time a person encounters an allergen, it might not cause a problem. It can take days or even years to develop an allergy. The immune system takes its time learning to recognize the allergen. Then it has to make antibodies to attack it. Antibodies are proteins. They help the body fight off serious threats, such as bacterial or viral infections. IgE — short for immunoglobulin (Ih-MU-no-GLOB-yu-lin) E — is the type of antibody that causes allergic reactions.

IgE antibodies activate chemicals that produce inflammation in the body. That inflammation triggers the symptoms of allergies. These can run from a stuffy nose and watery eyes to an itchy rash or a throat that threatens to swell closed. How inflammation shows up depends on where in the body that inflammation occurs. Inflammation in the skin might look like a rash. In the sinuses, it might cause a stuffy nose. In the gut, inflammation can cause vomiting or diarrhea.

IgE antibodies aren’t always bad. They help the body fight off parasites. Parasites are organisms that live off host organisms, often making them sick. Early humans were routinely exposed to parasites. They wouldn’t have survived without IgE antibodies.

People still have IgE antibodies. But these days IgE doesn’t have much to do because most people rarely encounter parasites. (There are parts of the world where parasites are common, however, and there these antibodies come in handy.) IgE antibodies are like a well-trained army with no more enemies to fight. Sometimes they get jumpy and attack the wrong thing. IgE antibodies helped our ancestors survive. Today they’re more likely to cause allergic reactions.

Allergies are pretty common. If you don’t have allergies, you probably know someone who does. The American Academy of Allergy, Asthma & Immunology estimates that in the United States alone, more than 50 million people suffer from one or more allergies.