Allergies start when your immune system misidentifies a harmless substance, like pollen or a food protein, as a threat and builds a defense against it. This process happens in two distinct phases: a silent “learning” phase where your body memorizes the substance, and a reactive phase where it attacks on every future encounter. Understanding both phases explains why you can be fine with something for years and then suddenly react to it.
The Two Phases of an Allergic Response
The first phase is called sensitization, and you won’t feel a thing while it happens. When an allergen enters your body (through your airways, skin, or gut), immune cells stationed near those surfaces detect it and carry it to your lymph nodes. There, they present the allergen to a type of white blood cell called a T cell, essentially training it to recognize this substance as dangerous. These T cells then release chemical signals that instruct another set of immune cells, B cells, to produce a specific type of antibody called IgE. Each IgE antibody is custom-built to latch onto that one allergen.
Once produced, IgE antibodies attach themselves to mast cells, which are packed with inflammatory chemicals and sit in your skin, airways, and gut lining. At this point, you’re “sensitized.” You have no symptoms, but your body is now armed and waiting.
The second phase is what you actually feel. The next time you encounter that allergen, it binds to the IgE antibodies already sitting on your mast cells. This triggers the mast cells to burst open in a process called degranulation. Within seconds, they dump histamine and other inflammatory compounds into surrounding tissue. That’s what causes the sneezing, itching, swelling, and mucus production you associate with an allergic reaction. A second wave of newly formed chemicals follows within minutes, and a third wave of inflammatory signals can continue for hours, which is why allergy symptoms sometimes linger long after the allergen is gone.
Why Some People Develop Allergies and Others Don’t
Genetics plays the largest role in determining whether your immune system is prone to this kind of overreaction. If neither of your parents has allergies, your baseline risk of developing them is about 15 percent. If one or both parents have allergies, that risk jumps to 30 to 60 percent. Having a sibling with allergies raises your risk to 25 to 35 percent. What’s inherited isn’t an allergy to a specific substance but rather an immune system that’s more likely to produce IgE in response to common environmental proteins.
But genetics alone doesn’t explain the dramatic rise in allergy rates over the past few decades, which is where environment comes in. The hygiene hypothesis, one of the most well-supported theories in allergy research, proposes that modern sanitation has left many children’s immune systems under-educated. Bacteria that naturally live in and around us train the immune system during infancy by activating a molecular switch on T cells called TLR4. When babies are exposed to enough bacterial molecules early in life, their immune systems learn to respond proportionally to real threats. When that training is lacking, T cells are more likely to overreact to harmless substances and trigger allergic pathways instead of protective ones. Studies consistently show that allergic diseases and asthma are more common in homes with lower levels of bacterial compounds.
This helps explain why children raised on farms, with pets, or in larger families tend to have lower allergy rates. Their immune systems got a broader education early on.
When Allergies Typically Appear
Many allergies first show up in early childhood, but the type of allergy matters. Eczema and food allergies often appear first, sometimes within the first year of life. Respiratory allergies to things like pollen, dust mites, and animal dander typically develop a bit later, once a child has had enough cumulative exposure for sensitization to occur. Some children follow what allergists call the “allergic march,” progressing from eczema in infancy to food allergies, then to asthma and hay fever in later childhood.
But allergies are not exclusively a childhood condition. You can develop a new allergy at any point in your life. Food allergies are actually more likely to newly emerge in adulthood than seasonal allergies. Moving to a new region and encountering unfamiliar pollen, changes in your living environment, and shifts in immune function over time can all trigger first-time sensitization in adults. If you’ve never reacted to shrimp or tree pollen before and suddenly start, it simply means your immune system completed the sensitization process for the first time, possibly after years of uneventful exposure.
The Role of Epithelial Barriers
Your skin, airways, and gut lining aren’t just passive walls. They actively communicate with your immune system. When these barriers are damaged (by dry air, pollution, certain chemicals, or conditions like eczema), cells in the lining release alarm signals that push nearby immune cells toward an allergic-type response. This is one reason eczema in infants is such a strong predictor of later food allergies: a compromised skin barrier may allow food proteins to enter the body through the skin rather than the gut, where they’re more likely to be tolerated. The immune system encounters the protein in the wrong context and flags it as dangerous.
Some allergens make this worse on their own. Certain proteins in dust mites, cockroach droppings, and mold have enzyme-like activity that can directly damage epithelial cells, essentially punching tiny holes in the barrier and triggering the alarm signals that promote allergic sensitization.
Common Allergens by Category
Inhaled allergens are the most frequent cause of seasonal and perennial allergies. These include pollen from trees, grasses, and weeds, dust mites, mold spores, pet dander, and cockroach particles.
For food allergies, nine major allergens account for the vast majority of reactions in the United States: milk, eggs, peanuts, tree nuts (such as almonds, walnuts, and pecans), wheat, soybeans, fish, crustacean shellfish, and sesame. Sesame was added to the official list in 2021.
Contact allergens, which cause reactions through skin exposure, include nickel, latex, fragrances, and certain preservatives in cosmetics. These trigger a different branch of the immune response than food and respiratory allergies but follow a similar two-phase pattern of sensitization followed by reaction.
Early Introduction as Prevention
One of the most significant shifts in allergy prevention has been the move toward introducing common food allergens to babies early rather than avoiding them. Current guidelines from the National Institute of Allergy and Infectious Diseases recommend that high-risk infants (those with severe eczema, egg allergy, or both) be introduced to peanut-containing foods as early as 4 to 6 months of age. For these high-risk babies, allergy testing before the first introduction is strongly recommended to determine the safest approach.
Infants with mild to moderate eczema can generally start peanut-containing foods around 6 months at home, without prior testing. Babies with no eczema or existing food allergies can have peanut-containing foods introduced freely alongside other solids. In all cases, other solid foods should come first to confirm the baby is developmentally ready to eat. This approach is based on landmark research showing that early, regular exposure to peanut protein significantly reduces the risk of developing a peanut allergy, essentially teaching the immune system tolerance before sensitization can take hold.
The same principle is being explored for other major allergens, with growing evidence that early dietary exposure during the first year of life may help prevent food allergies more broadly. The goal is to let the gut, which is naturally inclined toward tolerance, encounter these proteins before the skin or airways do.