The Role of Eosinophils in Asthma: What To Know (Part 1)

Eosinophils play a significant role in the late-phase asthma attack. In certain situations, they may also contribute to 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. Please feel free to check out the links for definitions 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 a dust mite allergy. When exposed to dust mites, your immune system responds. Various chemical reactions occur to make your airways hypersensitive to dust mites.¹ Each time you're exposed to dust mites, your immune system responds 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 that is the case with 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 (IL-4), Interleukin 13 (IL-13), and Interleukin 5 (IL-5).²

Interleukin 4 (IL-4) and Interleukin 13 (IL-13) 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, sneezing, a 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.

Eosinophils

The third mediator, IL5, travels through your bloodstream. Its 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.³

Late-phase allergic asthma attack

There are certainly other immune cells involved in this, but the main one is eosinophils. And it's IL-5 that recruits them. IL-5 encourages immature eosinophils to mature rapidly. 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.⁴

So, now let's introduce you to eosinophils.

Eosinophils

They were first discovered in 1967 and were almost immediately 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. So, once eosinophils arrive in your airways, they release these granules. Each of these granules travels directly to the airway walls, where it causes 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 challenging 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.^4,5

How eosinophils contribute to asthma

What I described here sets the stage for late-phase inflammation. In my next post, I will explain eosinophil content 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 what you understand about eosinophils. Have you heard of them before? Let me know in the comments below!