It surprises me that it was not until I started working in asthma/allergy that I began to see a clear relationship between our health and the ecological environment. In clinical practice, I encounter patients with many different types of allergic reactions, whose symptoms range from mild to severe to anaphylactic, and the causes of these allergic reactions are often common things we encounter in our everyday lives, including plant pollens, animal hair, dust mites, food, and/or insect bites.
For most patients, allergy symptoms are burdensome and affect the quality of their lives. Patients with allergic rhinitis often say, “I’m miserable, I’m sneezing all day and I feel congested and uncomfortable at work. I can’t sleep. I’m willing to do anything to help me feel better.” They take a combination of medications for many years, along with using air purifiers and dust mite covers at home. There are also patients who dedicate 5-7 years of their lives to allergy shots with hopes of completely eradicating their allergies. Additionally, patients with a history of anaphylaxis to certain foods or insect venom carry their epinephrine pen with them at all times and take real caution when eating out or traveling.
But it is interesting that modern science is still unsure why humans have allergies. We know that there is a genetic component to it and the mechanism of what happens in the body when we have an allergic response has also been studied extensively. But the question of why our bodies mount an immune response to non-infectious pathogens continues to be a mystery.
This mystery piqued my curiosity, and I started looking into this question that allergists, immunologists, and biologists have tried to answer for decades. In the process, I read about the interesting role of evolution in the allergic response. According to Fitzsimmons et al. (2014), the prevailing theory is that the IgE antibody evolutionarily developed as a result of our human ancestors' struggle with multicellular parasitic infections, that the IgE protected our human ancestors from these infections. But because the molecules targeted by IgE (which appear in helminths) also appear in allergens, the Th-2 immune and eosinophilic response elicited by helminth infections also occur with exposure to allergens. Thus, according to this theory, the allergic response is an inappropriate anti parasite immune response.
Ruslan Medhitov, an immunologist from the Yale School of Medicine, challenged this theory. As he studied the effects of venom on mice, he proposed that the IgE hypersensitivity reaction did not develop as an immune response to parasites but actually as a protective immune response against venoms and noxious substances. Medhitov proposes that sensitive allergen recognition evolved as a way for our human ancestors to scan the environment, including air, food, water, and the land, for noxious substances from plants or animals. The allergic response signaled them to leave and avoid suboptimal environments. He also stated that the symptoms of the allergic response such as sneezing, coughing, and vomiting expelled toxins out of the body, and stopped the spread and entry of some toxins. And that the damage caused by allergens signaled the immune system to respond and not the molecule of the allergen itself. Thus, the hypersensitivity reaction did not evolve to be a “misdirected” immune response but was actually protective.
Reading about the theories of how our bodies evolved to develop a hypersensitivity reaction is quite fascinating. When working, I often don’t think about how wonderful and complex our bodies are; we have this amazing innate and adaptive immune system within us that senses the presence of pathogens and noxious substances in our environment, and constantly strives to balance and protect our physical bodies.
As Michael Gross states in “Why Did Evolution Give Us Allergies, ” the pathophysiology of allergies is quite complicated. It involves a person’s genetics, the environment, and a combination of molecules and immune cells that communicate with one another to produce various inflammatory symptoms. This complex pathogenesis of allergic disease makes it challenging to establish a single treatment therapy that would work for all individuals.
I am currently grateful for novel biologic therapies which target specific pathways of the allergic response to help patients with severe asthma, angioedema, and atopic dermatitis. But there is so much more to know about how our body interacts with the environment. As Gross states, perhaps “more interdisciplinary work connecting immunology to ecology and toxicology may be helpful in improving our understanding of allergies” and what “evolution was thinking when it gave us allergic responses.” But as we await further studies, I will continue to do my best to empathize and help patients with their current symptoms, with hopes of better targeted treatment in the future.
What do you believe about the evolution of allergies? Share your theories in the comments.
Joanne is a mother and NP from San Francisco, CA. She loves to write about the intersection of health, spirituality, feminism, and motherhood. A story's ability to inspire wisdom and self-reflection in others is what motivates her to write. She is a 2020–2021 Doximity Op-Med Fellow.