Morphology and Characteristics of Basophils
Basophils are the least numerous group of leukocytes. Because the total number of leukocytes is vastly outnumbered by erythrocytes, there is on average only one basophil per 100,000 red blood cells. Consequently, they are extremely difficult to spot in a standard blood smear and require careful scanning of a large area of the specimen.
A key diagnostic feature of these cells is the presence of large (0.5 to 1.2 µm in diameter) specific granules in the cytoplasm. When stained with Romanowsky-type stains, they acquire a rich violet-cherry hue. These structures fill the cell so densely that they almost entirely obscure («mask») the weakly lobulated nucleus.
The contents of basophilic granules determine their function:
- Histamine is a major inflammatory mediator. It promotes vasodilation and significantly increases vascular permeability.
- Heparin is a natural anticoagulant that acts as part of the body's anticoagulation system.
- Enzymes include proteases, peroxidase, and other active substances.
Physiology and Activation Mechanisms of Basophils
The plasma membrane of basophils features specific high-affinity receptors for immunoglobulin E (IgE). While circulating in the bloodstream, these cells are almost always coated with IgE molecules that bind to their surface, keeping them poised for antigen encounter.
When an antigen binds to the cell-anchored IgE, it triggers the release of granule contents—IgE-dependent degranulation. This can proceed via two pathways:
- Slow degranulation — mediates classic inflammatory responses, regulates blood clotting, and vascular permeability.
- Rapid (acute) degranulation — leads to the development of allergic reactions. This process can be localized (e.g., urticaria or asthma) or generalized (anaphylactic shock).
An interesting property of basophils is metachromasia, driven by their high heparin content. This phenomenon occurs when toluidine blue dye causes the granules to stain red or violet, contrasting sharply with the original color of the dye itself. Additionally, like other granulocytes, basophils are capable of phagocytosis.
Structure and Granules of Eosinophils
Eosinophils feature a characteristic morphology: their nucleus is typically bilobed (consisting of two segments). The cell cytoplasm is packed with oxyphilic granules that turn a prominent pink color when stained with eosin.
Eosinophil-specific granules are cylindrical and considered by biologists to be lysosomal derivatives. Their key ultrastructural feature is the presence of a central dense crystalline structure known as the crystalloid.
The chemical arsenal of eosinophils includes:
- Major basic protein (MBP) (localized within the crystalloid) — exerts potent antihelminthic, antiprotozoal, and antibacterial effects.
- Enzymes — histaminase (a specialized enzyme that degrades histamine), various peroxidases, and hydrolytic lysosomal enzymes.
Functions of Eosinophils: Antagonism and Defense
Eosinophils act as direct antagonists to basophils, providing anti-inflammatory and antiallergic effects. Attracted by histamine, they migrate (chemotaxis) directly to the site of inflammation. Once there, eosinophils execute a multi-step neutralization of mediators:
- They inhibit further histamine release from basophils.
- They actively adsorb, phagocytose, and inactivate already released histamine.
- They completely degrade it using the enzyme histaminase.
Leukotrienes undergo similar degradation, effectively halting the progression of the inflammatory response.
Eosinophils' second major task is antiparasitic defense. The cells purposefully target parasite larvae. By releasing major basic protein and perforins, they disrupt the tough parasite cuticle, leading to parasite death. Phagocytic activity is also present in eosinophils, although it is less pronounced than in neutrophils.