General characteristics of eosinophils and basophils
Eosinophils and basophils are granulocytic leukocytes that are best considered together because they are functionally interconnected: the degranulation products of some cells stimulate the activity and degranulation of others. Their main common feature is participation in allergic reactions and antiparasitic defense.
In the peripheral blood of an adult, eosinophils significantly outnumber basophils. The relative count of eosinophils is 1–5%, while basophils account for 0–1%. Basophils are so rare that they may be entirely absent in a blood smear; their primary tissue-resident form is the mast cell. Knowledge of relative and absolute values is essential for interpreting the hemogram in clinical departments, as reference ranges are not always provided in examination blood tests.
Morphological differences
Under a light microscope, the cells are easily distinguished by granule color and nuclear appearance:
- Eosinophil: brightly stained oxyphilic (eosinophilic) granules, with the nucleus clearly segmented into two lobes.
- Basophil: large, intensely basophilic granules that obscure the nucleus, which is often impossible to visualize.
On an electron micrograph, an eosinophil is unambiguously identified by the presence of a crystalloid within its specific granules. Basophils and mast cells are characterized by metachromasia: when stained with toluidine blue against a blue tissue background, they appear bright violet due to their glycosaminoglycan content.
Composition of specific granules
The key components in eosinophil granules are:
- Major basic protein (MBP) — forms the crystalloid, accounts for granule eosinophilia, and exhibits potent antihelminthic, antiprotozoal, and antibacterial effects.
- Eosinophil neurotoxin — toxic to parasites and nervous system cells.
- Histaminase — an enzyme that degrades histamine.
- Peroxidase and perforins — induce osmotic shock and parasite death.
The granules of basophils and mast cells contain:
- Histamine — a mediator of inflammation and allergy.
- Heparin and chondroitin sulfate — glycosaminoglycans responsible for metachromasia.
- Chemotactic factors for eosinophils and neutrophils.
Mechanism of the allergic reaction
An allergic reaction is triggered by contact with an allergen, which induces the production of immunoglobulin E (IgE). IgE molecules opsonize mast cells and basophils by binding to specific receptors on their surface. Upon re-exposure to the same antigen, IgE cross-linking and mast cell degranulation occur.
As a result of degranulation, histamine is released into the tissue, increasing vascular permeability and causing soft tissue edema and skin hyperemia. This explains the clinical manifestations of allergic rhinitis, urticaria, atopic bronchial asthma, and life-threatening angioedema with an asthma attack.
Simultaneously, chemotactic factors are released, attracting eosinophils. Eosinophils degranulate at the site, releasing histaminase, which degrades excess histamine and limits the reaction. However, along with histaminase, major basic protein, neurotoxin, and peroxidase are also released, exerting a toxic effect on the host's own tissues: the bronchi in asthma, the nasal mucosa in rhinitis, and the skin in urticaria.
Antiparasitic function of eosinophils
During helminthic invasion, IgE antibodies bind to the parasite's surface. The eosinophil plasma membrane features Fc receptors for IgE, ensuring recognition of the opsonized target. The eosinophil attaches to the helminth surface and directionally releases granule contents: perforins damage the parasite's integument and cause osmotic shock, while major basic protein and neurotoxin complete its destruction. Because these toxic substances are not strictly selective, host tissues in the reaction zone are also damaged.
Functional interaction of cells
Eosinophils and basophils form a functional pair with mutual regulation:
- Basophils release histamine and eosinophil chemotactic factors.
- Eosinophils migrate to the lesion and inactivate histamine via histaminase.
- Histamine breakdown products additionally attract new eosinophils.
- Eosinophil degranulation is always accompanied by the release of the entire set of substances, resulting in local tissue damage during allergies.
Key points
- Eosinophils and basophils are granulocytes sharing a common function in allergy and antiparasitic defense.
- An eosinophil has a bilobed nucleus and a crystalloid in its granules; a basophil has large basophilic granules that obscure the nucleus.
- Eosinophil major basic protein exhibits antihelminthic, antiprotozoal, and antibacterial activity.
- Eosinophil histaminase degrades histamine, limiting the allergic reaction.
- Basophils and mast cells contain histamine, heparin, and chondroitin sulfate, and exhibit metachromasia with toluidine blue.
- Allergies develop via IgE-mediated mast cell degranulation with increased vascular permeability.
- Toxic components of eosinophil granules also damage the host's own tissues at the reaction site.
- Basophils and eosinophils mutually regulate each other's activity through chemotactic factors and degranulation products.