Allergies are rampant! I think everyone I know has one type of allergy, whether it skin related, upper respiratory or food! What is going on? Why is the prevalence of allergies exploding? If you want to know more, read below….
Although the term allergy is used loosely to refer to any reaction as an allergy, the medical community defines an allergy as a precise cascade of biochemical reactions. In genetically predisposed (or atopic) individuals, these reactions result in very specific symptoms. Atopy refers to the tendency for asthma, eczema and hay fever, characterized by an overactive immune response to environmental factors (Stanway, 2004). Symptoms include sneezing, wheezing, bronchoconstriction and even anaphylaxis (Rakel, 2018).
Much research has been focused upon the modification of the allergic immune response early in life, either during the prenatal period or early childhood. Atopy may be genetic, although environmental factors are thought to play a strong role in the development of atopy. Some examples include prenatal and postnatal infection exposure, timing of food introduction, and household pet exposure as predictors of atopy later in life (Rakel, 2018). The antecedents I would like to focus on are related to environmental factors.
Environmental factors interact with genetic factors to maintain allergic (TH2) immune responses, which activate eosinophils, promote IgE production which drive up allergy symptoms (Delves, n.d.). Interestingly, early childhood exposure to bacterial and viral infections and endotoxins may shift the immune response that can discourage allergic responses (Delves, n.d.). Trends in developed countries toward smaller families with fewer children and cleaner indoor environments are often thought to contribute to a phenomenon known as the “hygiene hypothesis”. This hypothesis proposes that excess ‘cleanliness’ in an infant’s or child’s environment can lead to a decline in the number of infectious stimuli that are necessary for the proper development of the immune system (Stanway, 2004). This includes early use of antibiotics that may limit children’s exposure to the infections that drive the opposite immune responses that drive down the allergic reponse, which can explain some of the rise of atopic allergies. Other factors can include chronic allergen exposure, diet and environmental pollutants.
The hygiene hypothesis also indicates that the early microbial environment is also essential for proper development of protective antimicrobial and regulator immune responses to environmental antigens (Liu, 2015). For example, parents who orally “cleaned” their pacifiers were less likely to have allergic sensitization, eczema and asthma by 18 months (Liu, 2015). Globally children who have older siblings demonstrate less risks of hay fever and eczema, which were more strongly correlated in more affluent countries (Liu, 2015). Interestingly, having older siblings may also influence the microbiome. “The infant gut microbiome associated with having older siblings was recently shown in a birth cohort to have more Bifidobacterium, Lactobacillus, Escherichia, and Bacteroidetes genera and less Clostridia” (Liu, 2015).
In another example, a house dust microbiome that is rich and diverse in the first year of life was negatively associated with allergic sensitization and atopic recurrent wheezing at age 3 years, possibly due to the abundance of Firmicutes and Bacteroidetes phyla. The same trend has been demonstrated in a household that has dogs.
In laboratory models, oral exposure to extracts of house dust from homes with dogs prevented the development of allergy associated asthma while altering the gut microbiome, including enrichment of Lactobacillus johnsonii (Liu, 2015).
Another area that has been given attention lately is the relationship between allergic diseases and helminth infections. Yes, its a type of worm! It has been demonstrated that helminths can induce a strong immune response while consequently being inversely associated with allergy and asthma! These include inducing dentritic cells to induce IL-10 producing Treg lymphocytes while inhibiting allergic airways inflammation in laboratory models of asthma (Liu, 2015). On the flip side, the innate immune system can also affect immune response against helminth infections which can induce inflammation. “This reaction is the body’s response to destroy helminths, especially tissue helminths that stimulate an excessive immune response that can promote allergies. Though inconclusive, it does demonstrate the role the microbial environment has on atopic allergic responses.
Delves, P. (n.d.) Overview of Allergic and Atopic Disorders. Retrieved (2018, May 22) from https://www.merckmanuals.com/professional/immunology-allergic-disorders/allergic,-autoimmune,-and-other-hypersensitivity-disorders/overview-of-allergic-and-atopic-disorders
Liu, A. (2015). Revisting the hygiene hypothesis for allergy and asthma. J Allergy Clin Immunol 2015;136:860-5.
Rakel, D. (2018). Integrative Mecidine (Vol. 4): Elsevier Inc.
Sitcharungsi, R., & Sirivichayakul, C. (2013). Allergic diseases and helminth infections. Pathog Glob Health, 107(3), 110-115. doi:10.1179/2047773213y.0000000080
Stanway, A. (2004). Causes of atopic dermatitis. Retrieved (2018, May 22) from https://www.dermnetnz.org/topics/causes-of-atopic-dermatitis/