Worldwide, allergies are on the rise at an alarming rate. How do our bodies mistake otherwise harmless substances for potential dangers and cause the unpleasant, and sometimes even fatal, symptoms of allergy?

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Allergies affect millions of people worldwide, and the number is rising.

From the mother anxiously watching for signs of wheezing the first time her child eats peanut butter to the retiree's sudden reaction to shellfish, allergies can strike at any point during our lives.

Hay fever affects 400 million individuals globally, with asthma affecting 300 million, food allergies between 200 and 250 million, and drug allergies affecting around 10 percent of the world's population.

The World Allergy Organization (WAO) warn that "the prevalence of allergic diseases worldwide is rising dramatically in both developed and developing countries."

Allergens, or molecules with the potential to cause allergy, are everywhere in our environment. They come in the form of tree pollen, food, mold, dust mites, snake or insect venom, and animals, such as cats, dogs, and cockroaches.

When the body mistakes one of these substances as a threat and reacts with an immune response, we develop an allergy. Nobody is born with allergies. Instead, the 50 million people in the United States who suffer from allergies developed these only once their immune systems came into contact with the culprit.

But how do our bodies mistake a friend for a foe? And what causes the symptoms that many are so familiar with?

Allergy is defined as an inappropriate immune response to an otherwise harmless substance in the environment.

Lisa A. Reynolds and B. Brett Finlay – both from the Michael Smith Laboratories at the University of British Columbia in Vancouver, Canada – explain in an article published in the journal Nature Reviews Immunology how the immune system reacts to foreign substances.

Our immune cells are always on the lookout for dangers, such as bacteria, viruses, parasites, and toxic substances. When these molecules enter the body – through the lungs, mouth, intestine, or skin – the immune system can react by labeling them as either harmless or dangerous.

Most of the time, our bodies accept or tolerate the presence of allergens.

This is called a Type 1 immune response, and the cell type at the heart of this process is the regulatory T cell.

That being said, in some individuals, the body's immune cells see the allergen as a threat, and a pro-inflammatory response occurs as a result. This is called a Type 2 immune response, and a different class of T cell appears on the scene: T helper type 2 cells.

These cells stimulate the production of immunoglobulin (Ig) E molecules in most allergies.

The first exposure to an allergen that results in a Type 2 immune response is called allergic sensitization.

Importantly, once the body has been sensitized, it maintains a lasting memory of the substance. And then, when it next comes into contact with the culprit, IgE molecules are primed to release a cascade of inflammatory players such as histamine, causing the unpleasant and potentially deadly symptoms of allergy.

Allergies can manifest in several different ways, and everyone's experience is unique. Our bodies can react by developing eczema (atopic dermatitis), hay fever (allergic rhinitis), allergic asthma, food allergies, or anaphylaxis, which is a severe and potentially deadly allergic reaction.

Allergies are a lifetime companion, and treatment mostly revolves around the management of symptoms.

But, as scientists are steadily getting to the bottom of what makes our immune system switch from Type 1 to Type 2 responses, there is a call to focus on preventing allergies from occurring in the first place.

With 40 to 50 percent of schoolchildren worldwide sensitized to one or more allergens, preventing allergies in the future is likely to have a huge impact on global health.