Allergies develop when your immune system mistakenly treats a harmless substance, like pollen or peanut protein, as a dangerous invader. This process isn’t instant. It happens in stages, starting with an initial exposure that “primes” the immune system, followed by increasingly aggressive reactions on future encounters. About 6 out of every 100 Americans have food allergies alone, and rates of all allergic conditions have been climbing for decades. The reasons trace back to genetics, gut bacteria, childhood environment, and even climate change.
What Happens Inside Your Body
The first time you encounter an allergen, nothing dramatic happens. Your immune system quietly takes note. Certain immune cells flag the substance as a threat and begin producing a specific type of antibody called IgE. These IgE antibodies then attach themselves to the surface of mast cells, which are stationed throughout your skin, gut lining, airways, and other tissues. At this point, you’re “sensitized” but you won’t feel a thing.
The second (or third, or tenth) time you encounter that same substance, things escalate. The allergen latches onto the IgE antibodies already sitting on your mast cells, cross-linking them like a key fitting into a lock. This triggers the mast cells to release a flood of chemicals, most notably histamine, along with enzymes and other inflammatory compounds. Histamine is what causes the familiar symptoms: itchy eyes, swelling, hives, a runny nose, or in severe cases, anaphylaxis. The whole reaction can unfold within minutes.
In the gut specifically, mast cells do more than just trigger immediate symptoms. They release signaling molecules that recruit additional immune cells to the area and reinforce the allergic response over time. This is partly why food allergies can worsen with repeated exposure. There’s even a skin-to-gut connection: allergen exposure through the skin (say, through cracked skin from eczema) can trigger a chain reaction of immune signals that causes mast cells in the intestines to multiply, potentially setting the stage for food allergies that seem unrelated to skin contact.
Why Genetics Stack the Odds
Your family history is one of the strongest predictors of whether you’ll develop allergies. If one of your parents has allergies, your risk roughly doubles compared to someone with non-allergic parents. When both parents are allergic, the risk climbs further, to about 2.3 times the baseline. This holds true across allergic conditions generally, not just for the specific allergy a parent has. A parent with hay fever might have a child who develops a food allergy or eczema instead.
That said, genes aren’t destiny. Plenty of people with allergic parents never develop allergies, and some people with no family history develop them anyway. The best way to think about genetic risk is as a loaded gun that still needs an environmental trigger to fire.
The “Too Clean” Problem
One of the most influential ideas in allergy science is the hygiene hypothesis, which suggests that growing up in extremely clean environments can leave the immune system poorly trained. The logic is straightforward: a developing immune system needs exposure to bacteria, viruses, and other microbes during infancy to learn the difference between genuine threats and harmless substances. Without that education, it’s more likely to overreact to things like dust mites or milk protein.
The evidence backs this up. Allergic diseases and asthma are more common in homes with low levels of bacterial molecules. Specifically, a bacterial compound called endotoxin acts as a training signal for immune cells. It flips a molecular switch on T-cells (a key type of immune cell) that helps calibrate the immune response during early life. Children raised on farms, in larger families, or in less sanitized environments tend to have lower allergy rates, likely because their immune systems got more of this early microbial education.
This doesn’t mean dirt is medicine or that hygiene is bad. It means the immune system evolved to expect a certain level of microbial contact during development, and modern lifestyles, with their antibacterial soaps, indoor living, and reduced contact with animals and soil, often fall short of that threshold.
Your Gut Bacteria Play a Bigger Role Than You’d Think
The trillions of bacteria living in your digestive tract appear to directly influence whether you develop allergies. Research from the American Gut Project found that adults with allergies, particularly to nuts and seasonal pollen, had significantly less bacterial diversity in their guts than non-allergic adults. The pattern was consistent: allergic individuals had fewer bacteria from a group called Clostridiales and more from a group called Bacteroidales.
This connection shows up early in life too. In one Swedish study, infants who had low microbial diversity in their stool at just one month old were more likely to develop eczema by age two. A Danish study found similar results: fewer bacterial species in infancy predicted allergic rhinitis by age six. The implication is that the composition of your gut bacteria during the first months of life may help determine your allergy risk for years to come. Factors that shape early gut bacteria, like mode of delivery (vaginal birth vs. cesarean), breastfeeding, antibiotic use, and diet, are all areas of active interest.
Climate Change and Air Quality
Allergy seasons are getting longer and more intense, and the changing climate is a direct contributor. Higher temperatures and elevated carbon dioxide levels stimulate photosynthesis and plant growth, which leads to more pollen production. The result is pollen seasons that start earlier, last longer, and pack higher pollen concentrations than they did a few decades ago. If your allergies feel worse than they used to, they probably are.
Why Some Adults Develop Allergies Out of Nowhere
It’s a common misconception that allergies are something you either have from childhood or don’t have at all. About 7 out of every 100 U.S. adults have food allergies, and many of them developed those allergies well into adulthood. Shellfish allergy is a classic example: because most people eat shellfish only occasionally, there’s limited opportunity for the immune system to build tolerance. That infrequent exposure pattern may actually increase the chance that the immune system will eventually misidentify it as a threat.
Moving to a new region can also trigger new allergies. You might live for years without reacting to a particular type of pollen simply because you were never exposed to it. Relocating to an area with different plant life introduces your immune system to new allergens, and sensitization can follow. Major life changes that affect the immune system, like pregnancy, illness, or significant stress, may also shift the balance toward allergic responses in people who were previously fine.
Early Food Introduction as Prevention
One of the clearest advances in allergy prevention has been the shift in guidance around introducing allergenic foods to infants. For decades, parents were told to delay foods like peanuts, but research showed this approach actually increased allergy rates. Current guidelines from the National Institute of Allergy and Infectious Diseases now recommend the opposite.
For infants at highest risk (those with severe eczema, egg allergy, or both), peanut-containing foods should be introduced as early as four to six months of age. These babies may need allergy testing first to determine the safest way to introduce peanut. For infants with mild to moderate eczema, introduction around six months is recommended, and parents can do this at home without a clinical evaluation. For babies with no eczema or food allergies, peanut-containing foods can be introduced freely alongside other solids whenever the family is ready.
The key detail: once introduced, peanut needs to stay in the diet regularly, about 6 to 7 grams of peanut protein per week spread over three or more feedings. Occasional exposure isn’t enough to build tolerance. Whole peanuts are a choking hazard for children under five, but peanut butter thinned into purees or dissolved peanut powder works well for infants.