An allergic reaction happens when your otherwise smart immune system overreacts to something usually benign, such as pet hair or pollen. And it's not just sneezing and a runny nose, either. "Allergic rhinitis, or hay fever, for instance, affects the nose and eyes, while eczema affects the skin," writes a team of allergy experts at The Conversation. "Food allergies affect the gut, skin, airways, lungs, and sometimes the entire body through the blood vessels."

The first signs of having an allergy usually show up in childhood, because that's when sensitisation tends to occur. In fact, allergies are the most common chronic childhood disease. But that doesn't mean we're completely out of the woods as adults.

Sensitisation refers to the first time your body encounters an allergen and makes the mistake of producing antibodies, which then bind to a type of white blood cell called a mast cell. The next encounter with said allergen can cause these mast cells to release infection-fighting substances - such as histamines - which is what creates the allergic reaction. 

Even if you've never struggled with allergies as a kid, what's really annoying is that they can - and do - pop up at any time during your life. Often this has to do with a change in environmental exposure to allergens.

"Say someone has a tendency toward allergies; for example, a person might get married and their spouse has cats," allergy specialist Edward Davis told Jennifer Scott at Everyday Health. "Cat dander is a very, very strong allergen. If you've never had indoor exposure to cats, it's very possible for you to develop an allergy to them."

The reasons for why people develop allergies are complex and varied, hence there's no one-size-fits-all explanation for why you suddenly swell up from nibbling on shellfish that you enjoyed as a kid. However, scientists have determined that there is often a genetic component.

The genetic predisposition to develop allergies is called atopy, and it runs in families - although it's not a guarantee that you'll be allergic. Conversely, children can grow out of allergies, too - it's not unusual for a toddler with terrible eczema or egg allergy to grow into teenage years and lose all their symptoms.

Meanwhile, moving to a different country or climate zone can trigger the development of an allergy. So can hormonal changes in women, or the sensitisation of your immune system as you age.

According to Making Sense of Allergies, a guide developed by the UK organisation, Sense about Science, there's no telling how many people who've been sensitised to an allergen actually go on to develop an allergy. "Studies looking at this question have investigated how many sensitised children go on to develop peanut allergy but they have come up with wildly different rates of 11 percent and 65 percent," they write.

One thing is clear, though - plenty of people who don't actually have allergies claim to have one, or to have acquired it. This leads to hyperbole in the way we perceive allergies and immunity, and also means people unnecessarily avoid certain foods, or take the wrong medications.

"There isn't one test to diagnose all allergies," warns Sense About Science. "Accurate diagnosis requires both a test and a medical consultation. Many allergy tests sold on the high street and online have no scientific basis and do not work, giving incorrect results that stop people getting the right treatment."