Allergies occur because your immune system misidentifies a harmless substance, like pollen or peanut protein, as a dangerous invader and launches a full defensive attack against it. This isn’t a one-time mistake. Your body first learns to react to the substance during an initial exposure, then overreacts every time it encounters that substance again. About one in three American adults has at least one diagnosed allergic condition, whether seasonal allergies, eczema, or food allergies.
How Your Immune System Learns to Overreact
The first time you encounter an allergen, nothing dramatic happens. You don’t sneeze, itch, or swell. But behind the scenes, your immune system is quietly building a weapon it didn’t need to build. Specialized immune cells encounter the allergen, process it, and decide it’s a threat. This triggers a class of white blood cells to produce a specific type of antibody called IgE, which is the same antibody your body uses to fight parasites. These IgE antibodies then attach themselves to mast cells, which are stationed throughout your skin, airways, and gut, essentially arming them for the next encounter.
This priming process is called sensitization, and it explains why you can eat shrimp or pet a cat dozens of times before suddenly developing a reaction. Your body was silently building its arsenal. The allergic reaction you eventually notice is actually the second phase of a process that started much earlier.
What Happens During a Reaction
When you encounter the allergen again, its proteins bind to the IgE antibodies already sitting on your mast cells. When two or more IgE molecules get cross-linked by the same allergen particle, it triggers the mast cell to rapidly dump its contents into the surrounding tissue. This process, called degranulation, releases histamine along with dozens of other inflammatory chemicals in a matter of seconds.
Histamine is the main driver of the symptoms you feel. When it binds to receptors throughout your body, it causes blood vessels to expand and become leaky, airways to narrow, mucus production to spike, and nerve endings to fire itch and pain signals. That’s why allergies produce such a wide range of symptoms: sneezing and a runny nose when histamine hits your nasal passages, hives when it reaches your skin, wheezing when it affects your lungs.
Symptoms typically begin within 10 minutes of exposure. Congestion and runny nose can persist for over 10 hours after a single allergen exposure, with nasal blockage usually being the most persistent and severe symptom. This prolonged response involves a second wave of immune cells that arrive later, sustaining the inflammation well after the initial histamine burst fades.
Why Only Some Substances Trigger Allergies
Not every protein your immune system encounters becomes an allergen. The substances that do trigger allergies tend to share certain structural features. Allergens are proteins or glycoproteins with specific three-dimensional shapes that create surface patterns your immune system’s antibodies can latch onto. Most of the spots where antibodies bind are formed by the way the protein folds in three-dimensional space, bringing together amino acids that are far apart in the protein’s original chain.
Dust mite allergens, for example, include proteins that function as enzymes capable of breaking down tissue. This enzymatic activity may itself help alert the immune system, essentially poking holes in the protective barriers of your airways and skin. Food allergens work somewhat differently. Because cooking and digestion partially break down protein structures, food allergens tend to have simpler, more linear binding sites that survive processing. A substance is classified as a “major allergen” when more than 50% of people allergic to that source react to it.
Why Some People Get Allergies and Others Don’t
Genetics plays a significant role. The tendency to produce excessive IgE in response to common environmental proteins, a trait called atopy, runs strongly in families. Children whose parents both have allergies face a fivefold increased risk of developing early-onset allergic conditions like eczema compared to children without that family history. But genes alone don’t explain the picture. Allergy rates have risen too quickly over the past few decades to be driven by genetic changes, which take many generations to spread through a population.
The environment you grow up in matters enormously. One of the most well-supported explanations is that modern sanitized living disrupts the balance between your immune system and the microbes it evolved alongside. Children who grow up with domestic animals, who are exposed to diverse bacteria early in life, and who encounter a wider range of microorganisms tend to develop stronger immune tolerance. Protective microbes in the gut and on the skin appear to train the immune system toward tolerance rather than overreaction, helping it learn which substances are genuinely dangerous and which can be safely ignored.
This explains a pattern in the data. In the United States, adults in rural areas are actually more likely to have seasonal allergies (28.1%) than those in cities (24.8%), possibly reflecting differences in pollen exposure. But food allergies and eczema are more common in metropolitan areas (6.8% and 7.9%) compared to rural areas (5.9% and 6.4%), which aligns with the idea that urban environments may offer less of the microbial diversity that helps calibrate the immune system during childhood.
When the Reaction Becomes Dangerous
Most allergic reactions are uncomfortable but not life-threatening. Anaphylaxis is the exception. It occurs when mast cells throughout the body degranulate simultaneously rather than just locally, flooding the entire system with histamine and other inflammatory chemicals at once. Blood vessels dilate so dramatically that blood pressure drops. Airways constrict severely. Multiple organ systems can fail.
Mast cells can also be triggered through pathways that don’t involve IgE at all. A receptor called MRGPRX2 on mast cells can be activated directly by insect venom components, certain medications, and neuropeptides, bypassing the usual IgE-mediated process entirely. This is why some people experience allergic-type reactions to substances they’ve never been exposed to before, and it’s part of why anaphylaxis can sometimes seem to come out of nowhere.
Why Allergies Persist
Once your immune system has been sensitized to an allergen, it maintains that memory through long-lived immune cells that continue producing IgE for years or even a lifetime. Each subsequent exposure reinforces the pattern: more IgE is produced, more mast cells get armed, and reactions can become more severe over time. This is also why allergies sometimes seem to appear in adulthood. You may have been slowly sensitizing to a substance for years before crossing the threshold where your body produces enough IgE to trigger noticeable symptoms.
The reverse can also happen. Some children outgrow food allergies as their immune system matures and develops tolerance. Seasonal allergies, on the other hand, tend to be more persistent, partly because repeated annual exposure to pollen keeps refreshing the immune memory that drives the reaction.