September 1st, 2009 by
It is not the allergen per se which causes damage, but the body’s abnormal response to it is catastrophic. This is true of all allergic disorders, but never more so than in the case of anaphylaxis. There seems to be another factor at play here, an ‘accelerator’ of some sort, which drives the allergy to frightening extremes. We don’t know what this accelerator is, but we know that it’s there, and we know that it’s lethal.
To understand this more fully, we will need to know something about the underlying mechanisms involved. To this end, let us start with the ‘final common pathway’, and work back-wards. The final common pathway, in this context, refers to a series of events which take place in the body during an anaphylactic reaction — whatever the cause of the reaction. We come yet again to our old friend (or enemy, as the case may be), the mast cell. And we meet, for the first time, its cousin the basophil. Simply put, the basophil is a mast cell which floats freely in the bloodstream. Both of these cells contain numerous potent chemicals. When they burst (degranulate) they release their potent load into the surrounding tissues and bloodstream. Once emptied, they can produce new chemicals at a ferocious pace, thus ensuring a continuous reaction.
‘But hold it right there!’ you may say, ‘Isn’t that what happens in allergic asthma, rhinitis, urticaria, angioedema, and so on?’ And, of course, you would be right. That’s exactly what happens in all type 1 forms of allergy. The difference in anaphylaxis is the ‘accelerating factor’ alluded to above. Let me illustrate this. Kevin is a ten-year-old boy with a history of asthma. His symptoms are well controlled because he takes his medication regularly and he has, with a little help, figured out his allergic and non-allergic triggers. His only other problem is that he is allergic to eggs. If he eats egg two days in a row he gets hives (urticaria). If he eats egg one day, avoids it the next, and eats it again on the third day, he gets no hives. The skin and blood tests confirm that he is sensitive to eggs, but he has never had a major reaction to them.
Okay, so Kevin has an allergy. Now compare that with another ten-year-old boy, Damien. He has no history of asthma, nor of any other allergic disease for that matter. He came to the Allergy Clinic because he broke out in hives and vomited violently after eating an egg. His mother assumed he was allergic to eggs and kept him away from them. Damien needed no encouragement to comply with this restriction, for he noticed a tingling rash on his face whenever eggs were being fried in the vicinity! However, some tine later he again broke out in hives, vomited, and this time started to wheeze. He was puzzled because he hadn’t eaten, or even been near, eggs — but he had just eaten fish fingers, and, although he didn’t know it, fish fingers contain egg.
Damien has anaphylaxis. He has something other than what we might call ‘a simple allergy’. He has an accelerating mechanism at work in his system which drives a fulminant reaction throughout his entire system (that’s why we call them systemic reactions’). This happens whenever he comes across even minute amounts of allergen Think about it. He comes across the smell of eggs in the frying pan and he gets symptoms. We are talking molecules of allergen here, not milligrams or ounces. Yet these few molecules an start a devastating cascade of allergic events throughout his system. You think I’m exaggerating? At the time of writing, an unfortunate visitor to a London restaurant collapsed with fatal anaphylaxis as a waiter walked past with a sizzling fish dish. He was known to be allergic to fish. That’s what I mean by an accelerator, and that’s what makes anaphylaxis so deadly.
Come back for a moment to the mast cell and basophil. The release of their potent load gives rise to the symptoms of allergy. As we have seen, if they degranulate in the nose we get rhinitis, in the chest we get asthma, in the skin we get urticaria, and so forth. However, if an accelerator is present, the cells degranulate throughout the entire system, and not just where allergen has reached. This is how it happens:
1. Something triggers the mast cells and basophils.
2. They degranulate, pouring their potent chemicals into the tissues and bloodstream
3. These chemicals have a direct effect on blood vessels, making them dilate.
4. Dilated blood vessels are leaky’, and allow fluid to escape from the bloodstream into the soft tissues and other organs, including the liver, intestine, lungs and brain.
5. These chemicals also exert a direct effect on smooth muscle making it contract.
6. Smooth muscles in the bronchial tubes, intestine and womb go into spasm.
The symptoms and signs of anaphylaxis arise directly from the above:
• hives and swellings in the skin (urticaria and angioedema) and/or
• sneezing (and other symptoms of rhinitis) and/or
• watery eyes (and other symptoms of conjunctivitis) and/or
• wheezing (and other symptoms of acute asthma) and/or
• difficulty taking a breath and/or
• hoarseness of voice and/or
• abdominal pain and/or
• vomiting and diarrhoea (which may be bloody) and/or
• anxiety, fainting and convulsions and/or
• irregular heartbeats, heart attack and, ultimately, cardiac arrest
Anaphylaxis is so deadly because of its effects on the airways, the heart and the brain. Let’s take a closer look at these.
The airway in anaphylaxis
The airway may be threatened at several levels: (i) the throat and voice box, (ii) the bronchial tubes and (Hi) the lungs themselves.
(i) Space is limited in the throat and voice box. When the swellings of angioedema occur in these places they can easily obstruct the airway. Indeed, sometimes the airway is completely obstructed. The symptoms of obstruction include a sensation of swelling in the throat, difficulty taking a breath, difficulty swallowing, drooling from the mouth, hoarseness of speech, noisy breathing and, when obstruction is complete, absolute inability to breathe.
(ii) Anaphylaxis affects the bronchial tubes in an asthmatic sort of way. They swell, go into spasm and produce thick mucus — all of which narrows the airway. The symptoms include cough, wheeze and shortness of breath. The progression of the asthmatic attack in anaphylaxis may be extremely rapid.
(iii) Finally, the lungs themselves are affected, as indeed all internal organs. The leaky blood vessels allow fluid to pour out of the bloodstream and into the alveoli. Thus, the lungs drown in their own fluids.
The heart in anaphylaxis
The heart may also be affected in several ways;
(i) Mast cells in the heart muscle burst, causing swelling and disruption of the heart’s normal rhythm.
(ii) The blood flow to the heart may be jeopardised, resulting in a heart attack.
(iii) The heart comes under further strain as the blood pressure falls. It may now beat erratically, sometimes stopping altogether — cardiac arrest!
The brain in anaphylaxis
One of the first ‘brain’ symptoms of anaphylaxis is a feeling of impending doom. The patient becomes anxious — even before the other symptoms of ‘allergy’ are manifest. Falling blood pressure may then cause a feeling of faintness, and more profound drops in pressure lead to loss of consciousness. Swelling in the brain may cause convulsions and coma.
The end result of unhalted anaphylaxis is a complete (metabolic) collapse of the system and death.