Eosinophils and Asthma: What To Know (Part 1)

Last updated: August 2022

Eosinophils play a significant role in the late phase asthma attack. When certain situations are present, they may also play a role in severe asthma and allergic asthma symptoms. Here’s all you need to know about eosinophils and their role in asthma.

Eosinophils and allergic asthma symptoms

There are various approaches I could use to introduce you to eosinophils and their role in asthma. I think I will just stick with allergic asthma, mainly because that is the most common and well-understood asthma subgroup. I will probably also segue somewhat into eosinophilic asthma, as eosinophils obviously play a significant role in that asthma subgroup as well.

Note: I do not want to take up a bunch of space with definitions. So, please feel free to check out links for definitions of terms and more information. And, of course, come back here when you're done.

Okay, so let's move on.

Early phase allergic asthma attack

So, let's say you have allergic asthma. Let's also say you have an allergy to dust mites. When exposed to dust mites, your immune system responds. Various chemical reactions occur to make your airways hypersensitive to dust mites. Each subsequent time you're exposed to dust mites your immune system will respond by causing airway inflammation. And it's this inflammation that causes asthma and allergy symptoms.

Abnormal immune response

Keep in mind here that the intended role of this response is to keep your airways free and clear of germs. It is not meant to trap and kill allergens. But, thus is the case when you have allergic asthma. In this way, the entire allergy and asthma response is an abnormal immune response.

Airway inflammation

So, you're exposed to dust mites a second time. This causes the early phase asthma attack. During this early phase, various mediators of inflammation are released from Th2 cells and mast cells. The three that cause inflammation in allergic asthma are Interleukin 4 (IL4), Interleukin 13 (IL 13), and Interleukin 5 (IL5).

Interleukin 4 (IL4) and Interleukin 13 (IL13) immediately travel to airway cells and cause airway inflammation. So, they cause allergic asthma symptoms within seconds of exposure to dust mites. Allergic asthma symptoms include chest tightness, sneezes, runny nose, and maybe some shortness of breath and coughing. Symptoms during this early phase are generally considered mild. They are easily reversed by getting away from the trigger or with treatment


The third mediator, IL5, travels through your bloodstream. It's role is to recruit reinforcements.

The reinforcements I'm referring to are eosinophils. And it's these reinforcements that play a key role in the late phase asthma attack.

Late phase allergic asthma attack

There certainly are other immune cells that are involved in this, but the main one is eosinophils. And it's IL5 that recruit them. IL5 encourages immature eosinophils to mature in a hurry. And, about 4-8 hours after you are exposed to your asthma trigger (dust mites in this case), they start to arrive in your airways.1

So, now let's introduce you to eosinophils.


They were first discovered in 1967, and almost immediately were thought to play a role in allergies and asthma. They are specialized white blood cells that hold over 200 granules within their walls. Each granule is a pro-inflammatory mediator of inflammation. So, once eosinophils arrive in your airways, they release these granules. Each of these granules travel directly to airway walls and cause inflammation.

Eosinophilic inflammation

Here's an important note about eosinophilic inflammation: The type of inflammation they cause is much more aggressive than early phase inflammation. This makes eosinophilic inflammation more difficult to treat than early phase inflammation. Asthma symptoms may be more severe and more difficult to treat.

Another note: The intended purpose of eosinophils is not to respond to allergens at all. Remember, this is abnormal when this happens. The intended purpose is to respond to parasites. Only in those of us with a predisposition to allergies and asthma do they respond to harmless allergens.1,2

How eosinophils contribute to asthma

What I described here pretty much sets the stage for late-phase inflammation. In my next post I explain the contents of eosinophils and how they contribute to asthma. The goal here is to give you a better understanding of the role of eosinophils in asthma. I will also explain how they contribute to severe asthma attacks and severe asthma, particularly of the eosinophilic subgroups. So, stay tuned!

In the meantime, tell me about your understanding of eosinophils. Have you heard of them before? Let me know in the comments below!

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This article represents the opinions, thoughts, and experiences of the author; none of this content has been paid for by any advertiser. The Asthma.net team does not recommend or endorse any products or treatments discussed herein. Learn more about how we maintain editorial integrity here.

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