What are Eosinophils?

Imagine your body as a large, busy city: blood vessels are the highways, organs are like buildings, and cells are the workers that keep everything functioning. The immune system is the body’s “defense force”, and eosinophils are an elite squad within that system. Eosinophils are constantly monitoring the body, ready to respond to threats, especially parasites [1]. Under a microscope, eosinophils look pink when stained with a special dye called eosin, which is how they got their name. They are typically 10–17 μm in diameter, so they are much smaller than most parasites they fight.

What are Parasites?

A parasite is an organism that lives on or inside another living organism, called the host. Parasites can range from microscopic sizes to over 30 cm long. Parasites take nutrients and shelter from the host without giving anything in return. They can cause illnesses and discomfort, especially when they infect the digestive system.

Common parasites include roundworms, which live in the intestines and can make your belly hurt. Tapeworms are flatworms that can grow very long inside a person’s body. Another type of parasite, called Giardia, is pear shaped and has tiny hair-like tails. These tails are called flagella, and they help Giardia swim through our bodies. These microscopic parasites are found in contaminated water, and they can cause digestive system problems [2]. There are also certain insect larvae (baby insects) that can infect the body as parasites, like tumbu flies or new world screwworms [3].

The scientific study of parasites is called parasitology, and experts in the field are called parasitologists. Parasitologists research how parasites live, spread, and affect their hosts. Their work helps doctors diagnose and treat parasitic diseases in humans and animals.

What do Eosinophils do?

Eosinophils have several important functions (Figure 1). To carry them out, eosinophils first need to be activated. Activation occurs when certain proteins, called antibodies, or chemical messengers known as cytokines, bind to their surface. Once activated, eosinophils attack parasites by surrounding the invader and releasing toxic granules in a coordinated assault. During allergic reactions, eosinophils respond to harmless substances like pollen or dust, which can cause sneezing, itching, or inflammation [4].

Eosinophils are great communicators. To carry out their functions, they release cytokines that either increase or reduce inflammation, depending on the situation [5]. To help repair tissues after an injury or infection, eosinophils release cytokines as well as growth factors. Together, these substances promote healing and tissue regrowth. Eosinophils do not work alone—immune responses are only successful because several different types of immune cells work together as a team. The cytokines eosinophils send are recognized by these other types of immune cells, helping different parts of the immune system to coordinate responses.

In short, eosinophils are incredibly versatile and critical to maintaining a balanced immune response. They are not just fighters—they are also messengers, cleaners, and healers.

How do Eosinophils Fight Parasites?

When a parasite is detected, the immune system releases cytokines to tell eosinophils to act. Eosinophils move to the infected area, where they surround the parasite. Other immune cells on the protection team release antibodies that attach to the surface of parasites. Antibodies are protective Y-shaped proteins that “tag” harmful invaders for destruction. Eosinophils use proteins on their cell surface, called receptors, to bind to those antibodies. Binding activates the eosinophil.

Once activated, eosinophils release extracellular traps, which are like sticky nets, to immobilize parasites [6], and they communicate with other immune cells to get them to join the response. Activated eosinophils also release toxic proteins to directly damage the parasite (Figure 2). Through this multi-step attack, eosinophils help eliminate the parasite and protect the infected person’s body from further harm. So, when a worm is hiding in someone’s intestines, lots of eosinophils arrive quickly, and the worm is destroyed. Eosinophils are small but mighty!

When Eosinophils Cause Problems

So far, you have learned that eosinophils are very important for immune defense, especially against parasites. Although they are usually helpful, problems arise when eosinophils become overactive or accumulate excessively in tissues, a condition known as eosinophilia. Eosinophilia can drive chronic inflammation and tissue damage, contributing to diseases such as asthma, allergic rhinitis (hay fever), eczema (dry, itchy skin), eosinophilic esophagitis (inflamed food pipe), and certain digestive system disorders. Eosinophils may also be involved in bad reactions to certain medications, autoimmune diseases (when the immune system mistakenly attacks healthy cells), and some cancers. The harmful effects result from the toxic proteins and inflammatory substances released by eosinophils, which damage tissues and disrupt normal organ function.

Conclusion: Eosinophils are a Vital Part of the Immune System

Eosinophils may be microscopic, but they play a powerful role in keeping your body healthy. Although they are well known as anti-parasite or allergy-associated cells, they also contribute to inflammation, immune regulation, tissue repair, and pathology (tissue damage).

Researchers are still trying to fully understand when eosinophils are protective vs. when they are harmful. Current research is looking at how different types of eosinophil work. For example, some spend their time in tissues (called tissue-resident eosinophils), and some promote inflammation. These are distinct types of eosinophils created by local signals. Researchers also need to better understand the role of these cells in parasite immunity, and what causes organ-specific eosinophilic diseases. They are trying to find better ways to spot and track eosinophils in the body and understand the long-term effects that can occur when they are not present.

By understanding the many roles eosinophils perform, we can better appreciate how the immune system works, and how even the smallest cells can have a big impact. So, the next time you hear the word “eosinophil”, remember—they are tiny cells busy keeping you healthy in many different ways!

Glossary

Immune System: ↑ A group of cells and organs that protect your body from sickness.

Eosinophils: ↑ A type of white blood cell that helps fight parasites, and repair tissues, but can also play a role in allergies and other dangerous responses.

Parasite: ↑ A living thing that lives on or inside another person or animal and feeds off it.

Host: ↑ The living thing a parasite lives on or inside.

Antibody: ↑ A protective Y-shaped protein produced by the immune system that binds to a specific protein called an antigen.

Cytokines: ↑ small proteins released by (mainly) immune cells that act as chemical messengers to regulate immunity and blood cell production.

Inflammation: ↑ Swelling and redness that happens when your body is trying to protect or heal itself.

Eosinophilia: ↑ a condition where a very high number of eosinophils are present in the blood.

Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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