What are allergies?

To unpack what an allergy actually is, and what’s going on in the body when someone has an allergic reaction, Chris Smith spoke to Eoin McKinney - an immunologist at Cambridge University. First of all Chris asked Eoin to explain exactly what an allergy is...

Eoin - Good question. There’s two key points to the answer: the first is what you would notice if you had one yourself and the second is what’s going on inside your body to cause it. It’s probably easier to start with the second one first.

It boils down to your immune system. And the point of your immune system is to repel invaders. To stop bugs, bacteria, viruses getting in and causing disease. And for bacterial and viruses that are extremely small, your immune system has cells that can effectively gobble up those pathogens and wave them around so that the rest of your immune system attacks them.

But sometimes there are pathogens that are too large and that can’t really happen to, so there’s a separate branch of your immune system that deals with that. The sorts of things we're talking about here are parasites or worms. There are a branch of cells, including specialised cells called mast cells stuffed full of granules and toxic proteins that they can release to repel or kill the invading worm or parasite.

Chris - And where are these mast cells?

Eoin - Throughout your body but they’re the sort of places you might expect if you were to plan a defence. So they’re lining your nose, they’re lining your lungs, they’re lining your gut, and they patrol around looking for trouble and trying to sort it out. When they see it, they release the proteins, repel or kill the invading bug and that should be that.

Chris - Right. So say some kind of parasite is trying to get through my skin it would engage with one of these cells - these mast cells - trigger it, a bit like a tripwire and it would then discharge things like histamine out of the skin and that would be why I would develop a local reaction like in the case we were saying hay fever, so that’s a pollen is hitting the skin, eyes, nose and throat and it’s causing the histamine to be released?

Eoin - It is indeed, you're absolutely right. In the case of a worm, you’d have all these things released and you would still notice inflammation. So it might hurt, it might go red and that’s partly due to these toxic chemicals being released but it’s doing you good.

Chris - But why is it doing me good? It doesn’t feel like it’s doing me good is what I’m getting at, so why is that beneficial?

Eoin - It would be beneficial if it was targeting a worm or a parasite. If it’s targeting something that’s a natural component of your environment, like grass pollen, like peanut, like egg. We could go on for the huge list of things, that’s not beneficial and you really don’t want that to take place; that’s an accident and that’s why it’s a disease.

Chris - Why is it that it’s confined in hay fever just to my eyes and nose? But if I’m allergic, we heard just now about a young person with a nut allergy you can get symptoms all over your body, so why is there that difference?

Eoin - It partly comes down to the severity of the response. An allergy is not like a binary on or off phenomenon, you can have it but it can be low grade. Or you can have it and it can be extremely severe and there’s a continuum the whole way along. The low grade stuff you get local reactions where the pollen coming in. It’s coming into your eyes, your nose and you have a local reaction there. If you have a much more severe reaction, such as to peanuts, it can affect the whole of your body. Not just small blood vessels dilate to give you a rash, they all do that. Your blood pressure can drop to your boots and it can actually be extremely dangerous or life-threatening.

Chris - Now you’re not born with an allergy, are you? Because your immune system is immature when you’re born and it has to learn from the environment what is good and what is bad. So how do allergies get started in the first place, do we know?

Eoin - The short answer is not really. You’re not born with an allergy but you can be born with a propensity to develop it.

Chris - So it can run in a family effectively and have a tendency towards an allergy?

Eoin - A tendency towards it can run in a family. It’s a risk so it’s not like there is a gene for allergy that you will inherit but you can be born with an increased risk for allergy. Now that’s great if that increased chance of an allergic type response helps you repel parasites and worms, but it’s not so good if it makes you react more against things you shouldn’t like grass pollen. So in some spectrums an allergic response can be helpful but in others not. It depends on the circumstance.

Chris - And in terms of managing allergy, obviously not being exposed to the thing you know triggers these symptoms is one way to manage it but that’s not always possible is it? If you’ve got to go about your day to day business and you’re reacting to pollen which is so tiny it’s just in the air all around you. So what’s the best way that a person can manage an allergic condition?

Eoin - Our treatment so far have largely been limited to trying to turn off these toxic chemicals like you mentioned histamines. And the cells are releasing histamine as a way of trying to agitate and get rid of the worm or parasite for example or, in this case, attacking the pollen, and we have things that can block that. So antihistamines are the drugs we take that simply try to stop the affective proteins that are driving the response.

Chris - So you pop a pill; that antihistamine drug goes round your system. How does it then stop, or how does it now where the allergic reaction going to happen so it knows to block it there?

Eoin - It doesn’t is the short answer. There’s no specific targeting of an antihistamine. It gets absorbed into your system, you’ve got it throughout your entire body and that’s why some of them have side effects that cause a bit of drowsiness as well because the affect other things too. It’s not as targeted or selective as we would like.

Chris - In essence, you're damping down your immune system a bit all round your body and that way you’re blocking the effect were it to happen in that particular area because the antihistamine is already there? So you’ve got to take it before you’re exposed, basically?

Eoin - No, not at all. In fact, as you know, everyone takes antihistamines after they’ve been exposed. The histamine is the protein that’s been released so it can still work when you take it after the response has started. In some instances, people take it beforehand to stop it starting or to limit it’s onset as well, but it’s never 100 percent effective.

Chris - And just very briefly, apart from antihistamines, any other drugs or classes of drugs that can be used to control allergies?

Eoin - Yes. There’s a class of drug called steroids. Now not the sort of steroids you might have associated with the Olympics, but the sort of steroid that can dampen down your immune response. They’re widely used for a huge range of different sorts of immune diseases because they dampen down the immune response a little bit, or a lot depending on the dose. Your immune systems a bit like a thermostat and what you want to do is turn it down a couple of degrees, not turn the heating off.