A peanut allergy is an immune system overreaction to proteins in peanuts, which can trigger symptoms ranging from mild hives to life-threatening anaphylaxis. Why this condition develops is complex, involving a mix of factors that cause the immune system to see peanut protein as a threat. Understanding this allergy requires looking at how a person’s inherited traits interact with their environment.
The Role of Genetics in Peanut Allergies
Genetics play a substantial role in determining susceptibility to a peanut allergy. Twin studies suggest the heritability of food allergies can be as high as 80%, indicating a strong genetic link. If an immediate family member, like a parent or sibling, has a peanut allergy or another allergic condition like asthma or eczema, a child’s risk is significantly higher.
This inherited risk is polygenic, meaning multiple genes contribute to the predisposition. Scientists have identified several gene regions associated with an increased risk, including the human leukocyte antigen (HLA) region. Genes in this area, like HLA-DR and -DQ, help the immune system distinguish between the body’s own proteins and foreign ones. Variations in these genes can cause the immune system to misidentify harmless peanut proteins as a danger.
Other genes, such as c11orf30/EMSY, are linked to a general allergic tendency, known as atopy, and are also associated with eczema and asthma. The presence of these genetic variants does not guarantee a peanut allergy will develop. Instead, they create a genetic predisposition that makes an individual more vulnerable when other factors are introduced.
Environmental Triggers and Influences
While genes create a predisposition, environmental exposures often act as the trigger for an allergy. One theory is the “dual-allergen exposure hypothesis,” which suggests the route of exposure to peanut protein in infancy is a determining factor. Exposure through the skin, particularly if compromised by a condition like eczema, may lead to sensitization, where the immune system learns to view the protein as a threat.
Conversely, introducing peanut protein orally through diet at an early age appears to promote tolerance, training the immune system to accept it as safe. The timing and method of this first encounter can set the course for either an allergic reaction or a normal digestive response.
The “hygiene hypothesis” suggests that cleaner living conditions reduce exposure to microbes needed to train the immune system, making it more prone to overreacting to harmless substances. The composition of an infant’s gut microbiome is also a factor. Differences in gut bacteria between allergic and non-allergic infants suggest that a healthy microbiome helps establish immune tolerance.
The Connection to Other Allergic Conditions
Peanut allergies are often intertwined with other atopic conditions. Many children who develop a peanut allergy first show signs of atopic dermatitis, or eczema. This progression is often called the “atopic march,” where eczema in infancy is followed by food allergies, and later by allergic rhinitis (hay fever) and asthma.
The link between eczema and peanut allergy is strong. Severe eczema damages the skin’s natural barrier, which can allow environmental peanut proteins, like those in household dust, to penetrate the skin. This skin-level exposure can trigger immune sensitization, laying the groundwork for an allergic reaction when the child later eats peanuts.
This connection also has a genetic basis. Mutations in the filaggrin gene (FLG) are a known risk factor for eczema because they impair the skin’s protective barrier. These same mutations are also strongly associated with an increased risk of developing a subsequent peanut allergy, highlighting a shared genetic vulnerability.
Assessing and Managing Risk
A child’s risk for developing a peanut allergy is assessed by understanding the combined influence of genetics and environment. Risk is evaluated based on two factors: a family history of allergic disease and the presence of eczema, particularly if it is severe and starts early. An infant with severe eczema and/or an existing egg allergy is considered to be at the highest risk.
This understanding has led to a shift in infant feeding guidelines. Research from the Learning Early About Peanut Allergy (LEAP) study showed that early and regular introduction of peanuts into the diets of high-risk infants, starting between 4 and 11 months of age, reduced the development of the allergy by more than 80%.
Based on this evidence, medical guidelines now recommend introducing peanut-containing foods to many infants around 6 months of age, but not before 4 months. For high-risk infants, this process should be done in consultation with a pediatrician or allergist, who may recommend allergy testing first. While genetic tests for risk-associated genes exist, they are not currently used for routine clinical diagnosis.