What causes allergies?
That’s a tricky question.
There’s more than one way to look at the causes of allergies.
We could be asking why some people suffer with allergies and others don’t.
Or we could be asking about the processes involved in allergic reactions.
The question of what causes some people to suffer with allergies is too complicated to answer fully here.
If you have allergies it may be due to a number of factors.
It seems to be due to heredity (if your parents have allergies you are more likely to), what your mother was exposed to while she was pregnant with you, the kind of gut bacteria you had as a child, and a whole lot of other things that aren’t very well understood.
The second question, about processes in allergic reactions, is a bit more straightforward. So we’ll look here at what happens in an allergic reaction.
An allergic reaction is just your body trying to protect you against something that isn’t really harmful, and it ends up causing you other problems instead.
Cells that trigger an allergic reaction
In our bodies we have cells called Mast Cells. They start off as a kind of white blood cell (called basophils) and then move from the blood to live in various parts of the body. These Mast Cells form an important part of our defences against infection.
These Mast Cells are found in very large numbers in parts of the body that are exposed to the environment – because that’s where the threats are going to be coming from.
Which parts of the body are exposed the environment?
The skin, of course, as the environment is all around us. Your nose and airways, because when you breathe you bring the environment (air) into your body. Your digestive system, because when you eat or drink something you bring the environment into your body in the form of food and liquids.
Mast Cells trigger inflammation
Mast Cells respond to trauma and infection by releasing chemicals that cause inflammation.
It’s fairly clear what infection means, but what about trauma? Trauma is any kind of damage to our bodies. If you get hit, cut yourself, break your leg, or whatever, that’s trauma.
You can see this for yourself if you rub the underside of your arm with your fingertips for a few seconds. This will cause inflammation.
With pale skin this can be seen as reddening of the skin. If you have darker skin the redness amy not be visible, but skin will look a little darker in that area.
Rubbing your skin for a little while longer causes it to become swollen.
This is inflammation.
In the example above the Mast Cells respond to the (fairly light) trauma by releasing chemicals such as histamine.
These chemicals cause the blood vessels in the skin open up (vasodilation) and become leakier (increased vascular permeability), causing the reddening and swelling that you see.
Something else these chemicals do is attract other white blood cells to the area to help fight infections and start wound healing. That’s one of the main purposes of inflammation.
Antibodies that attach to Mast Cells
The body’s defences against infection are very complex. Something you may have heard about the immune system and the body’s defences is the production of antibodies against bacteria and viruses.
These antibodies recognise and help to destroy the bacteria and viruses to rid our bodies of the infections.
After we have had an infection some of the antibodies remain in our bodies for a long time. These antibodies can recognise those bacteria or viruses from the previous infection, protecting us against those same infections in future.
So where would be a good place for some of these antibodies to be stationed on the lookout for future infections? Probably on cells stationed to watch out for infections coming in.
As I mentioned above, the cells in the immune system that are found in large numbers where your body is exposed the environment (e.g. skin, eyes, airways, digestive system) are Mast Cells.
So these antibodies are stationed on the Mast Cells in your skin, airways and digestive system, for example. The type of antibody that does this is called IgE, which is short for Immunoglobulin E.
Immunoglobulin is just another name for an antibody, so these would be E-Type Antibodies (any vintage Jaguar fans reading this?).
Antibodies on Mast Cells help us fight infection
Having these IgE antibodies attached to Mast Cells in skin, airways and digestive tract can help if you get an infection that these antibodies recognise. That is, an infection you’ve had before.
Since these Mast Cells are located at the first places the body would encounter an infection, having these antibodies attached can give the immune system a head-start on the infection.
The antibodies recognise the infection and cause the Mast Cells to release inflammatory chemicals.
This means you get an inflammatory response in that part of the body, which attracts the rest of the immune system to fight the infection.
Antibodies can also recognise harmless things
But what if these antibodies recognise something other than bacteria or viruses?
What if they recognised pollen, or dairy products, for example?
These allergy triggers are also known as allergens.
If the antibodies recognise pollen, then when you inhale pollen the Mast Cells in your airways would cause inflammation, and that would cause asthma. Asthma is inflammation of the airways caused by an allergic trigger.
Hay fever can also be caused by pollen, but this case the inflammatory response is in your eyes and nose.
If it was dairy products, the Mast Cells in your digestive system would cause inflammation there, and that could be the cause an inflammatory intestinal condition.
In addition to things you might inhale or eat, things that come into contact with your skin would cause allergic dermatitis (skin inflammation).
The most serious type of allergic reaction is anaphylaxis. This is a severe allergic reaction that affects several parts of the body all at once.
Identifying the trigger for allergic reactions
So how can you tell what triggers your allergic reaction?
It wouldn’t be very helpful to ask people with asthma to inhale various things that might be the trigger to see which one causes them to have breathing problems.
They way that allergic triggers are normally identified is with skin-prick tests. This is because the Mast Cells in your skin have the same antibodies attached to them, recognising the same triggers as those in your airways.
We’re using the example of a skin prick test for someone with asthma.
In this case, extracts of different types of pollen, animal fur, mould spores and dust mites are lightly pricked into the skin.
This brings the potential triggers into contact with the Mast Cell antibodies. The one that produces the biggest red and swollen reaction in the skin (the most inflammation) is the one that you are allergic to. This means that you can try to avoid this allergen in future.
We’ll have a look at treatments for allergies another time, and maybe even have a go at the subject of why some people produce these antibodies that recognise things that aren’t going to do us any (or much) harm.