Histamine – the enemy within for allergy sufferers

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You probably know someone who suffers from allergies that result in hayfever, asthma or urticaria. It could be you. The recent heatwave in the UK has caused huge discomfort for many such allergy sufferers. A period of rain allowing lush grass growth followed by the heat has given rise to a perfect storm of airborne pollen. Antihistamines are the main pharmaceutical remedy for allergies, and I imagine they are flying off the supermarket shelves as I write. Cetirizine, Loratadine, and Acrivastine are types of antihistamine readily available in the UK.

But have you ever wondered why ‘anti-histamines’? Why is histamine such bad news for allergy sufferers, and why do our bodies produce it if it wreaks such havoc? Well, actually, our bodies need histamine for normal functioning. Histamine plays an essential role in protecting the skin, airways and digestive tract from invaders such as parasites, bacteria and viruses. It also acts as a chemical messenger in the brain, helping to keep us alert. Additionally, histamine helps to stimulate the release of stomach acid. Histamine’s role in fighting the invasion of harmful organisms and initiating tissue responses to damage is what affects so many allergy sufferers.

Photo of airboorne grass pollen
Airborne grass pollen

Mediator of protection and healing

If you cut your finger or inhale a virus into your air passages, your body needs to supply extra blood to those areas to bring in the necessary ‘weapons’ and healing substances. Release of histamine, which is stored in ‘mast cells’ in the skin and mucous membranes, makes the tiny blood vessels dilate, allowing more blood into the area. The walls of the tiniest blood vessels, the capillaries, become more permeable to allow protective white blood cells and fluid to pass into the tissues. They become ‘leaky’. This leakiness means that tissues swell with extra fluid and blood cells. In the skin, this appears as swelling and redness. In the airways, there is congestion and watery mucus appears. Itching and sneezing can occur from stimulation of nerve endings.

Histamine molecule
Histamine is a small molecule made of three nitrogen, five carbon and nine hydrogen atoms.

Harmless substances taken for pathogens

When someone has an allergic reaction, their body interprets harmless substances such as pollen as being pathogenic. At some point in the past, their immune system has become sensitised. The hygiene hypothesis proposes that children in the developed world have insufficient exposure to a wide range of harmful organisms, and a relatively ‘idle’ immune system targets harmless substances instead. Allergic reactions are most closely aligned with the mechanisms for fighting parasitic invasions, in which there is a massive release of histamine in the affected tissues.

In allergic individuals, then, histamine feels like the enemy within. The itching, soreness, sneezing, coughing and wheezing evoked by histamine release cause extensive misery, and in extreme cases an allergic reaction can be fatal.

Anaphylaxis

In extreme cases, histamine release can be deadly. If you have a severe allergy to wasp or bee stings, certain medications, or foods, for example, it may cause anaphylaxis. Overwhelming levels of histamine are released into many parts of the body within a very short time. Skin breaks out in urticaria, the mouth and throat swell, airways constrict. Rapid leakage of fluid from the capillaries into these body tissues can cause a fall in blood pressure, leading to fainting. All these effects combine into anaphylactic shock, which is life threatening. The main lifesaver is an injection of adrenaline, which counteracts the most damaging effects of histamine. You may be interested in this informative video featuring Anaphylaxis Campaign Professor John Warner OBE:
https://www.youtube.com/watch?v=rlMYStReFL8

Mast cells

A discussion of hayfever feels quite mundane after contemplating anaphylaxis! It is worth taking a closer look at the mast cells before a final thought about antihistamines. Mast cells are made receptive to allergens (which act as antigens – something that triggers an immune response) by antibodies attaching themselves to the cell. Once triggered, the mast cell releases histamine into the surrounding tissues. Most of the time, this is a local response in the air passages or a certain place on the skin. In anaphylaxis, mast cells are triggered on a large scale.

A diagram of a mast cell
A mast cell releasing histamine (diagrammatic)

The blood-brain barrier

You remember I mentioned that histamine acts on the brain to help us stay alert? Presumably, then, antihistamines interfere with this? I remember as a child feeling privileged that my GP took the trouble to explain to me why the antihistamines I was taking made me feel sleepy. Luckily, for those hayfever sufferers nowadays who drive or operate machinery or are doing exams and so on, pharmacologists have developed modern antihistamines to minimise the effects on the brain. They have taken advantage of something known as the ‘blood-brain barrier’, identified in 1913. Modern antihistamines are barely able to pass from the blood into the brain through the blood-brain barrier, and therefore have almost negligible effects on a person’s wakefulness.

It’s quite a simple idea to grasp, although very complicated in detail, and still the subject of intense research. In short, the blood-brain barrier is a unique physical and chemical structure in the body that keeps the majority of the substances carried by the blood out of the brain. It will allow, for example, glucose, oxygen, amino acids, hormones and anaesthetics to pass through, but not antibodies, toxins and bacteria. Although alcohol can be considered a toxin, it is small enough to slip through the blood-brain barrier. If scientists could prevent alcohol passing into our brains, I wonder if this would increase or decrease our consumption?

I wish all you allergy suffers some respite from the sneezing, itching and wheezing this summer!

Photo of grass
A grass head laden with pollen
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Montgomeryshire Field Society visits Montgomery

Close-up photo of a bee visiting a Teasel flower
Bee on Wild Teasel

Now that the excitement of the spring and early summer flowers has passed and farmers are well on the way to harvesting their cereal crops, August has a mature feel about it. Dave and I have stopped exploring the hedgerows on foot as other demands have encroached on our time. Therefore, I was delighted to make an excuse to get out there with the Montgomeryshire Field Society this week.

The Montgomeryshire Field Society planned to explore Montgomery Castle and the surrounding countryside this August, as part of its Summer programme 2015. They have surveyed and reported on the flora and fauna in and around Montgomeryshire for over 60 years.

After meeting up in the very pleasant surroundings of Monty’s Brewery Visitor Centre for a picnic lunch (and having a taste of my favourite tipple, Monty’s Sunshine), we set off on a range of walking routes. The ‘birders’ took a route through Lymore park and along Offa’s Dyke (similar to this one), and some others opted to walk in and around the town. I joined the ‘long walk’ that took us past the castle, up to the county war memorial (‘the monument’) and around the fields between Montgomery and the village of Llandyssil.

A little group formed around the edge of the path leading to the monument. They weren’t looking at the brambles or the nettles or the hogweed heavy with seeds. No, they were studying the docks intensely. I have never paid all that much attention to docks before, apart from being thankful that you can usually find them near nettles, and that they do indeed soothe nettle stings. Now suddenly, they were attracting great attention and interest.

There were two species of dock in close vicinity of each other. The first was Wood Dock, Rumex sanguineus. Its latin name is far more descriptive than its English name, as ‘sanguineus’ refers to the blood red sap visible inside the stem, and sometimes in the leaf veins. A quick online search reveals that the Americans call it Bloody Dock and have cultivated varieties of it for their gardens, mainly as a foliage plant. The flowering spikes had turned an attractive red in the plants we looked at.

I didn’t get a photo of the other dock. This one was Broad-leaved Dock Rumex obtusifolius. This one is much more feared as a weed, as it is slightly poisonous and can harm livestock. The milky sap has a reputation for causing dermatitis. The flowering spikes were much more robust and ‘chunky’ looking.

On we went, stopping to look at plants in the extensive daisy family. Hawksbeards, plume thistles, and sow thistles dominated much of the terrain. One of the plants we identified early on was the Nipplewort Lapsana communis. It is a modest plant that finds it way easily into gardens and can be quite attractive in its own way. Perhaps it gets its name from the somewhat nipple-shaped flower buds, although some sources claim it is effective at healing sore nipples!

Photo of nipplewort flower
Nipplewort flower with 8 florets
Photo of nipplewort flower
Nipplewort flower with about 15 florets

Willowherbs also caught our attention. Close inspection of the flowers revealed that they had either a four-lobed stigma or a blunt club-shaped stigma. Using this information and the leaf size and shape and growing habits we identified Broad-leaved willowherb Epilobium montanum, Great willowherb E. hirsutum, Marsh Willowherb E.palustre, and possibly Short-fruited willowherb E. obscurum. We also observed the unmistakable Rosebay willowherb Chamerion angustifolium growing abundantly.

Close-up photo of willowherb flower
Willowherb flower showing stigma with four white lobes
Close-up photo of willowherb flower
Willowherb flowers showing a club-shaped stigma

Two other rather impressive plants, both in the daisy family, are worth a mention here. First, the Lesser Burdock Arctium minus.  This time of year when the flowers are bursting from their spiky buds, the intense pink can easily stop you in your tracks. Likewise, Wild Teasel Dipsacus fullonum takes on a special beauty as its tiny pale purple flowers create a halo around the unmistakable egg-like flower heads.

Photo of flowering Teasel head
Flowering Teasel head with bee

The Field Society members, who are very skilled at identifying plants and other wildlife, soon created a long list of species of interest. It was a great sunny day for butterflies. We spotted many Gatekeeper butterflies, and also Common Blue, Meadow Brown, Speckled Wood, Peacock, Painted Lady, and Comma butterflies.

If you are visiting Montgomery in August, there is plenty to observe out and about!