Allergies develop through a two-step process: your immune system first misidentifies a harmless substance (like pollen or a food protein) as a threat, then overreacts the next time it encounters that substance. About one in four American adults has a seasonal allergy, and roughly 6.7% have a food allergy. Understanding how this process works helps explain why some people get allergies and others don’t.
How Your Immune System Creates an Allergy
The first step is called sensitization, and it happens silently. When a substance like pollen, pet dander, or a peanut protein enters your body for the first time, immune cells at the surface of your skin, nose, or gut capture it and present it to other immune cells. Your body treats this harmless protein the same way it would treat an invading bacteria or virus. Through a chain of interactions between different types of immune cells, your body produces antibodies specifically designed to recognize that substance.
These antibodies attach themselves to mast cells, which are stationed throughout your skin, airways, and digestive tract. At this point, nothing happens. You feel completely fine. You can actually carry these armed mast cells for years, or even your entire life, without experiencing a single symptom. You may never know sensitization occurred.
The allergic reaction itself only happens on re-exposure. When you encounter the same substance again, it latches onto the antibodies sitting on your mast cells. This triggers the mast cells to burst open and release a flood of chemicals, including histamine. Histamine is what causes the familiar symptoms: swelling, itching, runny nose, hives, or in severe cases, anaphylaxis. The whole process from exposure to reaction can take as little as 15 minutes.
Why Some People Get Allergies and Others Don’t
Genetics play a significant role. If one of your parents has allergies, your risk of developing them roughly doubles compared to someone with no family history. If both parents have allergies, the risk climbs higher still. What you inherit isn’t a specific allergy to cats or shellfish but rather a general tendency for your immune system to produce those antibodies in response to harmless substances. You and your allergic parent might end up reacting to completely different things.
But genetics alone don’t explain the picture. Allergy rates have risen sharply in industrialized countries over the past few decades, far too quickly for genetic changes to account for. This is where environment comes in.
The Role of Early Microbial Exposure
One of the most well-supported explanations for rising allergy rates is sometimes called the hygiene hypothesis. The idea is straightforward: young children’s immune systems need exposure to bacteria and other microbes to develop properly. In very clean environments, the immune system doesn’t get enough of this training. Without it, certain immune cells fail to learn the difference between real threats and harmless proteins, making allergic responses more likely.
Epidemiological studies back this up. Allergic diseases and asthma are more common in homes with lower levels of bacterial molecules. Children who grow up on farms, around animals, or in larger families with older siblings tend to have lower rates of allergies. The bacteria that naturally live inside and on humans help educate the immune system by activating a molecular switch on immune cells. When that education is weak or absent, the immune system is more likely to misfire.
This doesn’t mean dirt prevents allergies or that hygiene is bad. It means that early, diverse microbial exposure helps calibrate the immune system so it responds proportionally to actual dangers.
Common Allergy Triggers
Nearly all allergic reactions are triggered by proteins. For food allergies, the nine most common culprits in the United States are milk, eggs, fish, shellfish, tree nuts, peanuts, wheat, soybeans, and sesame. These account for the vast majority of food allergy cases. There’s also a less common type called alpha-gal syndrome, triggered by a sugar molecule found in red meat like beef, pork, and lamb, which is linked to tick bites.
Environmental allergens include pollen (from trees, grasses, and weeds), dust mites, mold spores, pet dander, and insect venom. Seasonal allergies are most common in adults between ages 45 and 64, and women are more likely to have them than men (29.5% versus 20.7%). Food allergies trend in the opposite direction, becoming less common with age: 7.4% of adults aged 18 to 44 have one, compared to 4.7% of those 75 and older.
Can You Develop Allergies as an Adult?
Yes. While many allergies first appear in childhood, new allergies can develop at any age. Moving to a new region exposes you to unfamiliar pollens your immune system hasn’t encountered before. Changes in your immune system over time, illness, or hormonal shifts can all trigger sensitization to something you’ve been around for years without issue. Adult-onset food allergies are well documented, particularly to shellfish.
How Allergies Are Diagnosed
If you suspect you have an allergy, testing typically involves one of two methods. A skin prick test uses a thin needle to introduce tiny amounts of potential allergens into the skin of your forearm or back. If you’re allergic, you’ll see redness or raised bumps within about 15 minutes. A blood test measures the levels of allergy-specific antibodies your body produces in response to individual allergens. Your provider sends a blood sample to a lab, where it’s exposed to suspected triggers and measured for antibody levels. Skin tests give faster results; blood tests are useful when skin conditions or medications make skin testing unreliable.
Early Introduction May Lower Risk in Children
For parents wondering whether they can reduce their child’s allergy risk, the evidence on food allergies is clear: early introduction helps. Pediatric guidelines now recommend offering common allergenic foods like peanut products, egg, dairy, and sesame starting around 6 months of age, when a baby is ready for solids. For infants at high risk of peanut allergy (those with severe eczema or an existing egg allergy), introducing peanut-containing foods as early as 4 to 6 months is recommended. Studies show that delaying introduction of these foods actually increases the chance of developing an allergy, reversing decades of older advice that told parents to wait.