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Short answer: Allergy and atopy are today exclusively used to describe various types of immune dysregulation. Currently atopy is a category of allergy. It covers hay fever (allergic rhinitis), asthma and eczema (atopic dermatitis). Along with a predisposition for early onset, atopy is believed to also have a genetic predisposition, and to be characterized by high allergen-specific IgE.

However, the short answer is deeply unsatisfactory to me because the word allergy was originally coined for a truly revolutionary concept that got lost, and the word perverted to mean something else entirely. Yet, it is a remarkable concept that, if revitalized, could make us see immune responses in a different, perhaps more holistic, light. Indeed, it might even make us question the current definition of the “immune system” itself.

To elucidate the original concepts behind the words “allergy” and “atopy“, and to contextualize their usage today, let’s delve into a brief history of these two words.

Allergy

In 1906, a young Viennese pediatrician, Clemens von Pirquet coined the word “allergie“. His ambition was no less lofty than to define the essential features of an immune response. Accordingly, he defined allergy or rather allergie as an “acquired, specific, altered capacity to react“. According to Igea (The history of the idea of allergy), Von Pirquet‘s revolutionary insight was in conceptualizing the function of the immune system not as one that rids the body of disease but as one whose contact with an antigen changes the reactivity of an individual.

Von Pirquet‘s revolutionary concept was deeply misunderstood in his own time, and instead “became perverted and limited” to describe “hypersensitivity” reactions (The history of the idea of allergy). Over the past century, “allergy” was further misappropriated out of the clinic and absorbed into the immunology lexicon to instead convey “antipathy and rejection” (The history of the idea of allergy).

Sadly, today, allergy has become an “umbrella” term used to describe all manner of adverse immune responses including drug side effects, and adverse reactions to food and food additives (Allergy).

Atopy

In 1916, Cooke and Vander Veer (Human Sensitization) distinguished patients with multiple “natural” hypersensitivities from those sensitized “artificially” by injection. Many of the former had hay fever (allergic rhinitis), asthma and eczema (atopic dermatitis), and some suggestion of common family history of similar disorders. The familial history implied a genetic underpinning for these diseases.

In 1923, at the suggestion of Edward D. Perry of Columbia University, Arthur F. Coca and Robert F. Cooke (On the Classification of the Phenomena of Hypersensitiveness) further distinguished hay fever and asthma patients by coining the word, “atopy“, which they defined as “inherited human hypersensitiveness“. Thus, “atopy” was originally coined to describe this specific subset of “allergic” reactions.

In the 1970s, Jack Pepys (Types of allergic reaction.) and others showed association between atopic disorders and high levels of circulating allergen-specific IgE, and pushed for its current usage. Thus, today, “atopy” is used to describe the so-called “classic triad” of hay fever (allergic rhinitis), asthma and eczema (atopic dermatitis), assumed to have a genetic predisposition and to be characterized by high levels of circulating allergen-specific IgE.

The problem is that since the 1950s, there is accumulating evidence of individuals with clinical symptoms of hay fever (allergic rhinitis), asthma and eczema (atopic dermatitis) lacking high IgE and familial history (Allergy: the body as self-evidence) (The environment and risk factors for… [Curr Allergy Asthma Rep. 2005]).

Such assessments suggest that atopy may have been coined using subjective, poorly delineated criteria, and that its continuing usage serves to confuse and obscure our proper understanding of immune dysregulation.

What are the differences between atopy and allergic hypersensitivity ?

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