What are Eosinophils?
Eosinophils are bone marrow-derived granulocytes that remain in the bloodstream for a shorter period and reside primarily in tissues.
- The functions of eosinophils are multifaceted, including antigen presentation, peptide release, lipid, and cytokine mediators.
- However, these can also be part of malignancies and autoimmune diseases and connective tissue disorders.
- Eosinophils account for approximately 6% of total circulating leukocytes and can be found in tissues of the lung, thymus, mammary glands, spleen, and uterus.
- Eosinophils that leave the bone marrow have long been believed to be mature cells; however, recent studies have indicated that there are multiple tissue-specific cell subtypes that differentiate on the basis of cell surface marker expression.
- Eosinophils present in the circulation are recruited to physiological locations and inflammatory sites in response to eosinophil-specific chemokines and a variety of other stimuli.
- These cells remain located around the invading worms where their membranes are damaged by the activity of proteins released by eosinophils.
- Additionally, eosinophils are known to play an important role in asthma and allergy symptoms in areas where parasitic infections are less frequent.
- Eosinophils are also associated with certain medical conditions such as eosinophilia, where the number of eosinophils varies between 450-550 cells / µL in the bloodstream.
Structure of Eosinophils
- Eosinophils are granulocytes, measuring between 10 and 16 µm in diameter. Cells contain a segmented or bilobed nucleus where the nucleus: cytoplasm ratio is approximately 30%.
- An important morphological characteristic of eosinophils is the content of their cytoplasmic granules, which contain specific cationic proteins.
- Eosinophil-specific granules are the main distinguishing feature of cells. These granules contain a unique crystalloid core, a dense matrix surrounded by a membrane. The nucleus is responsible for the cardinal properties of eosinophils.
- The granules also contain different mediators such as proteins, cytokines, chemokines, and enzymes that can induce inflammation and tissue damage.
- The cytoplasm also contains Golgi bodies, endoplasmic reticulum, and mitochondria that function to produce proteins and ATP in the cell.
- The histological structure of eosinophils depends on the level of activation of the cells since the number of vesicles in the cells is only observed while the cell undergoes gradual degranulation.
- The plasma membrane of eosinophils contains numerous specific receptors for certain cytokines that regulate maturation and other physiological functions of the cell.
How Eosinophils work against pathogens?
- Eosinophils are formed from multipotent hematopoietic stem cells under the influence of soluble mediators and transcription factors.
- Mature eosinophils contain a molecular system that allows the attachment and fusion of vesicles to the cell membrane.
- Degranulation or release of proteins and other components within vesicles are released by regulated exocytosis. The coupling complex is composed of soluble N-ethylmaleimide-sensitive factor binding protein receptors present on the target membrane and vesicles.
- In addition to degranulation, eosinophils have also been associated with antigen presentation to T cells.
- The process generally occurs after exposure to the allergen, where the cell expresses the machinery for antigen presentation, as well as a full set of costimulatory molecules such as MHC II.
- The eosinophils present in the tissues migrate to the draining lymph nodes and finally reach the proliferation zone through a process that is independent of the eotaxin receptor CCR3.
- Antigen-presenting eosinophils also promote the proliferation of antigen-specific T cells.
- The process of eosinophil accumulation on a particular side of inflammation involves numerous sequential interactions that allow cells to adhere and transmigrate through the endothelium layer.
- Eosinophil adherence to the endothelial layer involves different pathways, including CD18-dependent pathways, adherence to E selection, and P selection.
- In the case of allergen-induced eosinophil recruitment, cell movement is dependent on CD4 + T cells and interferon-γ.
Functions of Eosinophils
The following are some of the functions of eosinophils
- The most obvious function of eosinophils is their role in host defense against parasitic infections.
- Furthermore, eosinophils are also known to release lipids, peptides, and cytokines that mediate inflammation and host defense.
- Some eosinophils also function as antigen-presenting cells, as they can be induced to express MHC class II proteins.
- Eosinophils are effector cells that release lipid mediators such as leukotriene C4, lipoxins, and PAFs that contribute to the acute manifestations of allergic or immune responses.
- Eosinophils also have the potential to regulate mast cell functions by releasing granular proteins and cytokines.
- Eosinophils release a panel of cytokines such as IL-2, IL-4, IL-6, IL-12 that are capable of promoting T cell proliferation and activation of Th1 or Th2 polarization.
- Eosinophils in the lungs have been recognized to contribute significantly to airway hyperresponsiveness and asthma.