An immune response aimed at the wrong target
The immune system normally protects the body from germs. An allergy is what happens when that defense is directed at something that is usually harmless, such as pollen, dust mites, pet dander, certain foods, insect venom or some medicines. The substance that triggers the reaction is called an allergen.
Most people react to none of these, or barely notice them. In an allergic person, the body has learned, usually without any warning signs, to treat the substance as a danger.
Sensitization: the first meeting
Allergic reactions typically begin with a quiet first stage called sensitization. When an allergen enters the body, some people’s immune systems produce a type of antibody called immunoglobulin E, or IgE, that is specific to that allergen. These antibodies attach to the surface of mast cells, which are found in tissues such as the skin, the lining of the nose and airways, and the digestive tract. Basophils, a type of white blood cells that circulate in the blood, can also carry IgE.
At this point there are usually no symptoms. The person is simply primed.
The reaction on later exposure
When the allergen shows up again, it binds to the IgE on the mast cells. This triggers the cells to release stored chemicals, with histamine being the best known. Histamine and other released substances cause blood vessels to widen and become leakier, make nerve endings itch, increase mucus production and can tighten muscles around the airways.
That chain of events accounts for many familiar symptoms:
- sneezing, a runny or blocked nose and itchy eyes
- hives, itching or swelling of the skin
- coughing, wheezing or shortness of breath
- stomach cramps, nausea or diarrhea after eating a trigger food
Antihistamine medicines work by blocking histamine’s effects, which is why they help with some symptoms but not all. Other medicines act in different ways, and a doctor or pharmacist is the right person to ask about which are suitable for a given person.
Mild to severe
Allergic reactions range widely. Many are limited to a runny nose or an itchy rash. Others can involve several parts of the body at once. Anaphylaxis is a rapid, severe, whole-body reaction that can cause swelling of the throat, difficulty breathing, dizziness or a drop in blood pressure. It is a medical emergency, and anyone who thinks they or someone near them may be experiencing it should seek emergency help immediately rather than waiting to see if it passes.
The same allergen does not always cause the same strength of reaction each time, and it is hard to predict from a mild reaction how a later one will go. That uncertainty is one reason people with known serious allergies usually work out a plan with their health care provider.
Common types of allergy
Respiratory or environmental allergies. Pollen, mold spores, dust mites and animal dander can cause allergic rhinitis, often called hay fever, and can contribute to asthma in some people.
Food allergies. These involve the immune system reacting to proteins in particular foods. Common culprits in many countries include milk, eggs, peanuts, tree nuts, fish, shellfish, wheat and soy, though the list varies by region.
Skin allergies. Contact allergies occur when skin touches a substance such as certain metals or plants and becomes inflamed, usually after a delay of hours to days. This is a different mechanism from the IgE reaction described above.
Insect sting and drug allergies. Some people react strongly to venom from stinging insects or to certain medicines, including some antibiotics.
Allergy versus intolerance
People often say they are allergic to a food when they have an intolerance instead. Lactose intolerance, for example, involves difficulty digesting milk sugar because of low levels of an enzyme. It can cause bloating and discomfort but does not involve IgE antibodies and is not an immune emergency. Telling the two apart matters, because the risks and management differ, and a health professional can help work out which one is happening.
Why do allergies develop?
Researchers do not have one complete answer. Genetics play a role, since allergic conditions often run in families. Environmental factors seem to matter too, including early-life exposures, where a person grows up and perhaps the makeup of their gut microbiome. One long-discussed idea, often called the hygiene hypothesis, suggests that reduced early exposure to microbes may change how the immune system develops. It is a topic of ongoing research, and scientists regard the picture as more complicated than a single explanation.
Allergies can also appear or fade at different life stages. Some childhood food allergies lessen with age, while others persist, and adults can develop new allergies.
How allergies are identified
Doctors typically start with a person’s history of symptoms and when they occur. They may suggest skin prick tests, blood tests that measure IgE, or supervised food challenges, depending on the situation. Tests alone do not settle the question, since a positive result can show sensitization without proving a person will have symptoms. Interpreting results is the job of a qualified clinician. Anyone trying to understand patterns in their own symptoms may find that learning about correlation and causation helps, since having symptoms after a meal does not always mean that the meal caused them.
Managing allergies in general terms
Approaches generally fall into three groups: avoiding the trigger where possible, using medicines to relieve symptoms, and, for some allergies, a treatment called immunotherapy that gradually retrains the immune response under medical supervision. Which of these applies depends on the allergy and the person. This article is background information, not medical advice, and anyone with symptoms or concerns should talk with a health professional.