The word 'allergy' was coined in 1906, and is used to describe people's immune system hypersensitivity to seemingly harmless substances in their environment.
Allergic reactions can be light - e.g. a mild skin rash - or life-threatening if it leads to anaphylaxis.
A huge catalogue of substances have been documented for their potential to cause allergic reactions. Common examples include pollen, nuts, sesame, shellfish, medicinal drugs of all kinds, dust mites, metals, milk, etc. etc. A crucial factor is that these same substances don't normally cause any problems for the majority of people.
The complex biological process involved in an allergic reaction was first described in the mid 1960s, and is dependent on an antibody class known as immunoglobulin E (IgE), along with T-cell and B-cell activations. (Ref.)
Although the allergic process is now well understood, the reasons why specific substances trigger such strong reactions in certain individuals are not generally known.
It's now certain that allergic reactions are strongly linked to genetics - identical twins very often have exactly the same allergies - but the ways in which particular genes somehow (very accurately) 'specify' a person's allergic trigger-substances are completely unknown.
Another unexplained aspect is that people can spontaneously develop an allergy to a substance which previously caused no adverse effects for them. It's also quite common for people to suddenly lose their allergic reaction, and become 'immune'. Yearly hay-fever (pollen) reactions, for example, often spontaneously stop for no apparent reason.
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