Eosinophils

Intracellular structures and granules

The cytoplasm of eosinophils contains several types of structures, each with its own unique secretion profile:

• Specific (secondary) granules: large formations (up to 1 µm). Their core crystallizes major basic protein (MBP), while the matrix contains eosinophil cationic protein (ECP), neurotoxin EDN, and peroxidase EPO. These proteins are toxic to helminths but can also damage the host's own tissues.
• Primary granules: contain Charcot–Leyden crystals (their base is formed by the enzyme lysophospholipase).
• Small granules: identified only in tissue forms of eosinophils. They contain acid phosphatase, arylsulfatase, and peroxidase.
• Lipid bodies: serve as a depot for enzymes (cyclooxygenase, lipoxygenase) and arachidonic acid for eicosanoid synthesis.

Surface markers and receptors

To identify eosinophils by flow cytometry and distinguish them from neutrophils, the markers CD9 and CD35 (complement receptor CR1) are used. The cell membrane also features:

1. Immunoglobulin receptors: for IgG (FcγRII/CD32 and FcγRIII/CD16) and IgE (FcεRII/CD23).
2. Cytokine receptors: for IL-5, IL-3, GM-CSF.
3. Chemokine receptors: specific receptor for eotaxins — CCR3.
4. Adhesion molecules: integrins (β1-, β2-, and β7-) for migration through the vascular wall.

Interestingly, eosinophils express MHC molecules of both classes (I and II), enabling them to function as antigen-presenting cells (APCs).

Mechanisms of immune defense

The primary way eosinophils combat pathogens is extracellular cytolysis. Phagocytosis is much less characteristic of them. Upon contact with a parasite, the cell releases the contents of specific granules (MBP, ECP, EPO), which embed into the helminth cell membranes, disrupting their integrity.

Antiviral protection is mediated by ECP and EDN proteins, which exhibit pronounced ribonuclease (RNase) activity. The bactericidal effect is achieved through a "respiratory burst" with the generation of reactive oxygen species, nitric oxide, and halogens.

Under pathological conditions, the MBP protein released by eosinophils activates mast cells and basophils, contributing to the development of allergic (Th2-dependent) reactions.

Migration and secretory activity

Normally, eosinophils spontaneously migrate from the blood into the lamina propria of the gastrointestinal mucosa, as well as into the thymus, uterus, and mammary glands (where they participate in morphogenesis during pregnancy and the estrous cycle).

Cells are recruited to foci of allergic inflammation by the action of chemokines (RANTES, eotaxins), IL-5, and leukotrienes. An activated eosinophil becomes a source of potent mediator secretion:

• Cytokines (IL-2–6, IL-8, IL-10, IL-12, IL-13, TNFα, IFNγ, TGFβ).
• Chemokines (eotaxin, RANTES, MIP-1α).
• Eicosanoids, neuropeptides, and platelet-activating factor (PAF).

Class and exam questions

What else to study on this topic

• Mechanisms of exocytosis and degranulation of eosinophil specific granules
• The role of eosinophils in positive selection of T cells in the thymus
• Synthesis of eicosanoids in lipid bodies: enzymatic pathways
• Interaction of eosinophils with mast cells in the development of allergy
• Involvement of integrins in eosinophil migration to the site of inflammation

Other topics in the "Innate Immunity" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.