Allergies occur when the immune system mistakenly identifies a harmless substance, known as an allergen, as a threat. When exposed, the immune system overreacts, producing antibodies that trigger inflammation and symptoms. Since the immune system is a dynamic network, its state of alert and tolerance changes throughout a person’s lifespan. This allows individuals to both develop new allergies later in life and naturally overcome existing ones.
Developing Allergies Later in Life
The immune system can become newly sensitized to substances it previously tolerated, a process often called adult-onset allergy. While allergies are often associated with childhood, about half of adults with food allergies report that at least one began in adulthood. This sensitization can occur at any point, with many individuals starting to experience symptoms in their 20s or 30s.
The most frequently reported adult-onset food allergies include shellfish (shrimp, crab, or lobster) and tree nuts. Environmental triggers can also cause new allergies; moving to a new geographic region introduces different types of pollen, mold spores, or dust mites. Drug allergies, commonly to antibiotics like penicillin or nonsteroidal anti-inflammatory drugs (NSAIDs), may also manifest for the first time in adult years.
Changes within the body can also drive this shift in immune recognition. Hormonal fluctuations, such as those occurring during pregnancy or menopause, can alter allergic sensitivity. A major illness, viral infection, or chronic stress can disrupt the immune system’s balance, causing it to become hyper-responsive. For example, an existing pollen allergy may lead to an allergic reaction to certain raw fruits or vegetables, known as Pollen Food Allergy Syndrome, due to cross-reactivity with similar proteins.
Allergies That Often Resolve in Childhood
The immune system frequently develops tolerance to certain allergens during early development. This resolution is common for food allergies that appear in infancy, as the body’s digestive and immune systems mature. The most frequent examples of allergies that remit are those to cow’s milk, egg, wheat, and soy.
The probability of resolution is high for these specific allergens, though the timeline varies. Over 90% of children with a milk allergy achieve tolerance by adulthood. Nearly half of children with an egg allergy may outgrow it by age two. This contrasts sharply with allergies to peanuts, tree nuts, and shellfish, which are generally considered lifelong conditions, resolving in only a small percentage of individuals.
Underlying Factors Driving Allergy Shifts
These lifelong changes are rooted in the balance of the immune system’s T-cells, specifically T-helper 1 (Th1) and T-helper 2 (Th2). A non-allergic response is associated with a dominant Th1 profile, which focuses on fighting infections. Allergies are characterized by a dominant Th2 response, which drives the production of Immunoglobulin E (IgE) antibodies responsible for allergic reactions.
The shift between these T-cell profiles is heavily influenced by environmental exposure, partially explained by the Hygiene Hypothesis. This theory suggests that a lack of early-life exposure to diverse microorganisms and infections prevents the immune system from developing a strong Th1 response. Without this microbial diversity, the immune system may default toward the Th2-dominant, allergic state.
The composition of the gut microbiome, the community of microbes in the digestive tract, is recognized as a major factor influencing this immune balance. Changes in the microbiome due to diet, antibiotic use, or lifestyle can affect sensitivity to allergens. External triggers like viral infections or prolonged stress can cause systemic inflammation that alters the immune environment, potentially tipping the Th1/Th2 scales and initiating or suppressing an allergic response.