Allergies and Allergic Reactions

Posted on : 30.07.2018

Every day you are exposed to millions of potential pathogens that want to call your body home.[1] A single bacterial cell, replicating once every hour, can amass an army of more than 16 million progeny by the end of the day. To defend against this onslaught, we’re equipped with an extraordinary, adaptive immune system that grows stronger with each battle. Unfortunately, due to its complexity and the sheer number of potential nasties, the system sometimes gets things wrong, and in some people that means allergies and allergic reactions.

Why do some people have allergies?

Most sources will tell you that allergies are caused by a combination of genetic and environmental factors. This is true, but recent research might have found a surprising clue to the genetic origin of allergies. It’s been shown that most of us are still carrying an average of about 2% Neanderthal DNA in our genes[2] due to cross-breeding that occurred approximately 50,000 years ago. Hidden among those lingering genes are three that boost the innate immune response.[3] This was helpful when ancient humans were migrating to brave new lands, where new and harmful pathogens lurked, but it may also have introduced a propensity for hypersensitivities.[4] So whether you’re susceptible to allergic reactions or not could depend on which Neanderthal genes you’re expressing.

How an allergic reaction works

All allergic reactions are hypersensitivity reactions, and have two distinct phases; sensitisation, and secondary exposure.


It all begins with the absorption of an allergen, which could be inhaled (pollen), swallowed (egg protein), injected (bee venom) or absorbed through the skin (nickel). Once inside, it’s picked up by your body’s customs agents, antigen-presenting cells. These cells are trained to recognise potential threats, but aren’t quite sure what to do with one when they find it, so they take it to the decision makers in your lymph nodes. Here, they present the allergen to T-helper cells. These microscopic detectives spend their lives looking for threats. Some are born and bred for the sole purpose of tracking down a specific antigen, even if they’ve never seen it before. When they get their hands on it, though, the T-helper cells become ‘primed’. Basically it’s just received evidence that its nemesis is out there, and it’s now a cell on a mission. That mission is simple: destroy the target at all costs.

The primed T-helper cell sounds the alert, releasing a molecule called interleukin-4, which recruits B-cells to start making IgE antibodies. The antibodies serve as wanted posters, which are handed out to the foot soldiers – Mast cells that roam the body.
All of this so far has been part of a covert operation, and you probably won’t have noticed a thing, but if the allergen dares show itself again, the Mast cells will unleash a devastating attack.

Secondary exposure[5]

It’s happened. The allergen has returned. Perhaps you’ve been stung by another bee, or gone back for more shellfish. Either way, it’s time for the Mast cells to do their thing, and they don’t mess around. When the allergen binds to two IgE antibodies on the outside of the cells, cross-linking them, it triggers a process called degranulation, where the Mast cell releases its payload in a chemical warfare attack. The weapon is histamine, and it has a number of effects on the body. First, it causes the blood vessels to dilate, and increases their permeability; in effect, it widens the highways and removes the safety barriers so the immune response cells can get where they’re going faster, and go off-road into the tissues. As fluid leaks into the tissue, we get redness and inflammation. Histamine also stimulates sensory nerves, causing itch. This can all unfold over the course of only a few short minutes.

When allergy becomes anaphylaxis[6]

The most serious case of allergy is anaphylaxis. It’s a case of the body’s defences gone wrong, where the body’s own adaptive immunological weaponry—designed to take out infection by deadly parasites, bacteria or viruses—is turned against a relatively harmless allergen like peanut protein.

Among histamine’s list of effects is the ability to cause smooth muscle contraction, which in the airway means difficulty breathing. When combined with the drop in blood pressure due to blood vessel dilation the vital organs can quickly be starved of oxygen. This is called anaphylactic shock, and the only way to reverse it is with an injection of epinephrine (adrenaline), which counteracts the effects of histamine.

Allergic reactions to skincare products

There are five different types of hypersensitivity reactions, and allergies due to chemicals on the skin, allergic contact dermatitis, are type-4 reactions.[7] They differ from the mechanism described above in that there are no antibodies involved. With type-4 reactions, the T cells do most of the dirty work themselves. Without the help of antibodies and Mast cells to spread the word, symptoms are slower to appear, usually taking around 2-4 days.[8] This can make it much more difficult to identify the culprit, especially since the resulting rash may not even be limited to the contact site but could appear anywhere on the body.[8] The most common causes of contact allergy are toxic plants, like poison ivy, but fragrance ingredients can also be responsible, as can preservatives like methylisothiazolinone.[8,9]

What to do about allergies

The best way to prevent an allergic reaction is to avoid the allergen that causes it, which is usually possible for food-based allergens, but not so easy when it’s something like pollen or dust mites. Anti-histamines may also be an option for some people. There are advances in ‘desensitisation’ and immunotherapy, where an immunity can be gradually built up by giving progressively higher and higher doses of the allergen, but it must be administered by a qualified professional, and clinical evidence is still lacking.[10] If you have sensitive skin or are prone to allergic reactions to skincare products, make sure you test any new products on a small patch of skin first. For our tips on patch testing cosmetic products, check out this article. There are many mysteries remaining when it comes to hypersensitivity. But continued research may one day shed more light on its causes, mechanisms and potential treatments.

1. Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P. Innate Immunity. In: Molecular Biology of the Cell. 4th edition. Garland Science; 2002.
2. Prüfer K, Filippo C de, Grote S, Mafessoni F, Korlević P, Hajdinjak M, et al. A high-coverage Neandertal genome from Vindija Cave in Croatia. Science 2017;358(6363):655–8.
3. Gibbons A. Our hidden Neandertal DNA may increase risk of allergies, depression [Internet]. Science 2016 [cited 2018 May 22];Available from:
4. Dannemann M, Andrés AM, Kelso J. Introgression of Neandertal- and Denisovan-like Haplotypes Contributes to Adaptive Variation in Human Toll-like Receptors. The American Journal of Human Genetics 2016;98(1):22–33.
5. Delves PJ. Hypersensitivity: IgE-Mediated (Type I). In: Encyclopedia of life sciences. Chichester: John Wiley & Sons, Ltd; 2017.
6. Lee JK, Vadas P. Anaphylaxis: mechanisms and management. Clinical & Experimental Allergy 2011;41(7):923–38.
7. Thomas WR, Cunningham PT. Hypersensitivity: Immunological. In: eLS. American Cancer Society; 2015. page 1–13.
8. Nosbaum A, Vocanson M, Rozieres A, Hennino A, Nicolas J-F. Allergic and irritant contact dermatitis. European Journal of Dermatology 2009;(4):325–332.
9. Schwensen JF, Uter W, Bruze M, Svedman C, Goossens A, Wilkinson M, et al. The epidemic of methylisothiazolinone: a European prospective study: A European Prospective Study of MI Contact Allergy. Contact Dermatitis 2017;76(5):272–9.
10. Lacombe S. Allergy – Policy briefing [Internet]. British Society for Immunology; 2017 [cited 2018 May 29]. Available from:

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