Morphology and features of basophils
Basophils are the least numerous group of leukocytes. Because the total number of white blood cells is thousands of times lower than that of red blood cells, there is on average only one basophil per 100,000 red blood cells. For this reason, they are extremely difficult to detect in a standard blood smear, requiring a careful examination of a large area of the slide.
A key diagnostic feature of these cells is the presence of large specific granules (0.5 to 1.2 µm in diameter) in the cytoplasm. With standard Romanowsky staining, they acquire a rich violet-cherry hue. These structures fill the cell so densely that they almost completely mask («camouflage») the weakly lobed nucleus.
The content of basophil granules determines their function:
- Histamine is a critical inflammatory mediator. It promotes vasodilation and significantly increases vascular permeability.
- Heparin is a natural anticoagulant that is part of the blood anticoagulation system.
- Enzymes — proteases, peroxidase, and other active substances are present in the granules.
Physiology and activation mechanisms of basophils
Specific high-affinity receptors for class E immunoglobulins (IgE) are located on the plasma membrane of basophils. While in the bloodstream, these cells are almost always coated with IgE molecules that adsorb onto their surface and remain ready to encounter an antigen.
When an antigen binds to the cell-bound IgE, stimulation of granule content release occurs — IgE-dependent degranulation. It can proceed via two pathways:
- Slow degranulation — ensures the development of a classic inflammatory response, regulates blood clotting and vascular permeability.
- Abrupt (rapid) degranulation — leads to the development of an allergic reaction. This process can be localized (e.g., urticaria or asthma) or generalized (anaphylactic shock).
An interesting property of basophils is metachromasia, caused by their high heparin content. The phenomenon lies in the fact that when using the toluidine blue dye, the granules stain red or violet, which is fundamentally different from the original color of the dye itself. In addition, basophils are capable of phagocytosis, like all other members of the granulocytic series.
Structure and granules of eosinophils
Eosinophils are distinguished by a characteristic morphology: their nucleus is typically bilobed (consisting of two segments). The cell cytoplasm is packed with oxyphilic granules that turn a distinct pink color when stained with eosin.
Specific eosinophil granules have a cylindrical shape and are viewed by biologists as derivatives of lysosomes. Their most important ultrastructural feature is the presence of a dense central lamellar structure called the crystalloid.
The chemical arsenal of eosinophils includes:
- Major basic protein (localized in the crystalloid) — exhibits potent antihelminthic, antiprotozoal, and antibacterial effects.
- Enzymes — histaminase (a specialized enzyme for degrading histamine), various peroxidases, and hydrolytic lysosomal enzymes.
Functions of eosinophils: antagonism and defense
Eosinophils act as direct antagonists of basophils, providing anti-inflammatory and antiallergic effects. Attracted by histamine, they migrate (chemotaxis) directly to the site of inflammation. Once there, eosinophils carry out a multistep neutralization of mediators:
- They inhibit the further release of histamine from basophils.
- They actively adsorb, phagocytose, and inactivate already released histamine.
- They finally degrade it using the enzyme histaminase.
Leukotrienes undergo similar destruction, effectively halting the progression of the inflammatory process.
The second critical task of eosinophils is anti-parasitic defense. The cells purposefully attack parasite larvae. By releasing major basic protein and perforins, they destroy the tough cuticle of the larvae, leading to their death. Phagocytic activity is also present in eosinophils, although it is less pronounced than in neutrophils.
Briefly: answers to topical questions
What is the normal percentage of eosinophils in the differential white blood cell count?
Normally in adults, the percentage of eosinophils in the differential white blood cell count is 2–4%. The leukocyte differential is the percentage ratio of various types of white blood cells, among which eosinophils are classified as granulocytes (granular leukocytes).
Normal ranges for other granulocytes in the formula:
- Neutrophils — the largest group: segmented (65–70%), band (3–5%), metamyelocytes (0–0.5%).
- Basophils — 0.5–1.0%.
Eosinophils act as «fire extinguishers» during allergies: they home in on the scent of histamine released by basophils and destroy it with histaminase, halting the inflammatory «fire».
Class and exam questions
What is basophil metachromasia and what causes it?
Metachromasia is the ability of granules to change the original color of a dye (for example, toluidine blue produces a purple or red coloration). This phenomenon is due to the presence of heparin in the granules.
By means of which substances do eosinophils destroy parasite larvae?
Eosinophils secrete perforins and major basic protein, which is located in the crystalloid of their granules. These substances effectively destroy the parasite cuticle.
How do eosinophils interact with histamine?
They migrate to the source of histamine, inhibit its further release by basophils, and adsorb and destroy it using the enzyme histaminase.
What else to study on this topic
- Mechanisms of IgE-dependent degranulation of basophils
- Differences between slow and rapid degranulation reactions
- Ultrastructure of the eosinophil crystalloid
- The process of eosinophil chemotaxis to the site of inflammation
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