What Is the Role of Eosinophils in an Allergic Reaction?

Eosinophils are a type of white blood cell that functions within the immune system. An allergic reaction is an immune response to a substance the body perceives as a threat. When the body encounters an allergen, these cells contribute to the symptoms associated with allergies. Understanding the connection between eosinophil activity and allergic reactions provides insight into how certain conditions develop and manifest.

What Are Eosinophils?

Eosinophils are a distinct type of white blood cell, classified as granulocytes due to the presence of small granules in their cytoplasm. These cells originate and mature in the bone marrow before entering the bloodstream and migrating to various tissues. Under normal conditions, eosinophils circulate in the blood at low levels, making up less than 5% of all white blood cells. They are most concentrated in tissues exposed to the external environment, such as the respiratory and digestive tracts.

Beyond their involvement in allergies, eosinophils have broader roles. They are recognized for their ability to combat certain infections, particularly those caused by parasites like helminths (worms). The granules within eosinophils contain proteins that are toxic to these invading organisms. They also participate in modulating immune responses and are involved in processes like tissue maintenance and repair.

The Allergic Reaction Process

An allergic reaction begins with sensitization, where the immune system is exposed to a harmless substance, such as pollen or a food ingredient, and misidentifies it as a threat. This triggers the production of Immunoglobulin E (IgE) antibodies. These IgE antibodies then attach to the surface of mast cells, which are abundant in the skin, airways, and digestive system.

Upon subsequent exposure to the same allergen, an immediate reaction is initiated. The allergen binds to the IgE antibodies coating the mast cells, causing them to degranulate. This degranulation is a rapid release of potent chemical mediators, including histamine, leukotrienes, and prostaglandins. These chemicals are responsible for the early-phase symptoms of an allergic reaction, such as swelling, itching, sneezing, and bronchoconstriction, which occur within minutes of exposure.

Eosinophils’ Role in Allergic Inflammation

During the late-phase allergic response, eosinophils are drawn to the site of inflammation by chemical messengers called chemokines. A key chemokine is eotaxin, which guides them from the bloodstream into affected tissues. Once there, the local inflammatory environment activates them, causing them to degranulate and release their own potent substances.

The granules inside eosinophils contain toxic proteins that contribute directly to tissue damage and inflammation. These include Major Basic Protein (MBP), Eosinophil Cationic Protein (ECP), Eosinophil Peroxidase (EPO), and Eosinophil-Derived Neurotoxin (EDN). MBP and ECP, for example, are highly effective at damaging tissues. This release of granule contents exacerbates the inflammatory state initiated by mast cells.

Activated eosinophils also synthesize and release other inflammatory molecules. They produce lipid mediators, such as leukotrienes, which constrict the airways and increase mucus secretion. Eosinophils also secrete cytokines, which are signaling chemicals that amplify the immune response by recruiting more immune cells. This sustained release of inflammatory agents is a primary driver of the chronic inflammation seen in many allergic diseases, leading to persistent symptoms like tissue swelling and airway hyperresponsiveness.

Common Eosinophil-Driven Allergic Conditions

Eosinophil activity is a prominent feature in several allergic diseases, where their accumulation in tissues is directly linked to symptoms. Eosinophilic asthma is a subtype of severe asthma characterized by high numbers of eosinophils in the airways. In these individuals, the eosinophils contribute to airway inflammation, swelling, and an increased risk of severe asthma attacks.

Atopic dermatitis, known as eczema, is another condition where eosinophils are frequently involved, and their infiltration into the skin contributes to the intense itching and inflammation. In the gastrointestinal tract, eosinophilic esophagitis (EoE) occurs when a large number of eosinophils accumulate in the esophagus, triggered by food allergens. This infiltration causes inflammation and injury to the esophageal tissue, leading to symptoms like difficulty swallowing, chest pain, and reflux.

Eosinophils also play a part in allergic rhinitis (hay fever) and chronic rhinosinusitis, particularly in cases involving nasal polyps. The presence of eosinophils in the nasal passages and sinuses perpetuates inflammation, leading to congestion and other persistent symptoms. Certain drug hypersensitivity reactions can also be driven by an eosinophilic response, where the cells contribute to widespread inflammation.

Diagnosing and Managing Eosinophilic Allergic Responses

Several methods are used to detect elevated eosinophil levels. A complete blood count (CBC) with differential is a blood test that can reveal an increased number of eosinophils in circulation, a condition known as peripheral eosinophilia.

For respiratory conditions, a sputum eosinophil count can directly measure the presence of these cells in the airways. In other cases, a tissue biopsy is necessary for a definitive diagnosis. For instance, an endoscopic biopsy of the esophagus is the standard method for diagnosing eosinophilic esophagitis (EoE). Biopsies of the skin or nasal passages can confirm eosinophilic involvement in atopic dermatitis or chronic rhinosinusitis.

Management strategies for eosinophil-driven allergies focus on reducing the inflammation caused by these cells. Corticosteroids, which can be administered topically, orally, or through inhalation, are broad anti-inflammatory agents that lower eosinophil numbers and inhibit their activity.

More recently, biologic therapies have been developed that offer a more targeted approach. These medications work by blocking specific molecules that are instrumental for eosinophil survival and activation, such as interleukin-5 (IL-5). Others work by targeting IgE antibodies, which can indirectly reduce the allergic cascade that recruits eosinophils.