Imagine you are stepping outside on a beautiful summer’s day, you admire the blue skies and take a deep breath in. Before you’ve even had a chance to put on your sunglasses, your nose is streaming, your eyes are itching, sneezes follow one after the other. What’s going on?
With even Manchester enjoying three (pretty much) solid days of sunshine it seems like summer has arrived, but for some this is a mixed blessing. An estimated 1 in 5 people in the UK suffer from hay fever (1), also called seasonal allergic rhinitis. In these people an allergic reaction to pollen can cause itchy eyes, blocked or runny noses and sneezing. In more severe cases, pollen allergies even trigger asthma attacks. If you are one of these unlucky people, I’m afraid your immune system is to blame. Allergies are caused by an immune reaction to something relatively harmless. The good news is that understanding how this works has helped provide some relief to those itchy noses, and may even reveal a cure for some allergies.
So what does happen after that first breath in the sunshine? Pollen from grass and trees is pulled in to your nose as you inhale. In hay fever sufferers white blood cells called mast cells, a type of granulocyte, are switched on by the pollen. This is happens because they are covered with antibodies which are made by B cells to stick to the pollen. Why B cells make lots of these antibodies in some people but not others is a question we’re still working on. There are a few possible reasons, but I’ll save that post for another day (it’s too interesting to cut down to a couple of sentences). Once the mast cells are in action they start releasing different chemicals, including histamine. Histamine makes your blood vessels leak fluid, causing the runny nose and watery eyes. It also makes blood vessels widen, letting in more blood and causing swelling, which leads to a blocked nose. The other chemicals act as an alarm, calling more immune cells to the scene. Specialised T cells called Th2 cells, which are seen in many allergic reactions, arrive and release more chemicals, cytokines. They can even start changing the cells that line your nose or lungs. This keeps the allergic reaction going for hours or even days.
You may have heard of histamine before, since most hay fever medicines you can buy over the counter are called anti-histamines. These drugs block histamine from affecting cells, relieving symptoms. So there’s one way understanding the immune system has helped with hay fever. Unfortunately anti-histamines don’t work for everyone. When all the different anti-histamines have failed we turn to steroids. Steroids are drugs which dial down the whole immune response, I’ve written about their use in auto-immune diseases before. Using steroids long term can have side effects, and they won’t work for everyone’s allergies. So what next? This is where we get to the exciting part. For very severe hay fever, pollen can be the cure as well as the cause.
Allergen immunotherapy means giving patients repeated of small doses of the trigger of their allergy. This can encourage the immune system to make regulatory T cells which shut down later immune responses to the allergen. The idea has been around for a long time. In 1911 Leonhard Noon and his colleagues started treating hay fever sufferers at St. Mary’s hospital with pollen injections (2) and found their symptoms improved. Since then researchers have found more effective ways to give allergen immunotherapies to patients, for example adding chemicals to give your immune system a boost making regulatory T cells, and moving from injections to pills. This type of therapy can also be used to treat allergies to cats and dogs, insect bites or stings and the dust allergies which cause many asthma attacks. Food allergies are a little trickier to treat because the immune cells in your digestive system behave differently to the rest of your body (another interesting topic for a post in the future). Recent studies have shown the allergen immunotherapy approach may be able to reduce peanut allergies in children, although we’re a long way from curing peanut allergies completely.
So if hay fever is dampening your excitement for summer, help may be at hand thanks to immune system research. Relax and enjoy the sunshine!