Eosinophils- Definition, Structure, Immunity and Functions

Eosinophils are a type of white blood cells that are involved in various immune responses, especially against parasitic infections. They are also associated with allergic reactions, asthma, and some inflammatory diseases. Eosinophils have distinctive granules in their cytoplasm that contain various proteins and mediators that can modulate inflammation and tissue damage. Eosinophils can also act as antigen-presenting cells and secrete cytokines that influence the polarization and activation of T cells. Eosinophils are derived from the bone marrow and circulate in the blood for a short time before migrating to different tissues, where they can differentiate into tissue-specific subtypes. Eosinophils are regulated by various cytokines, chemokines, and receptors that control their maturation, recruitment, and activation. In this article, we will explore the definition, structure, immunity, and functions of eosinophils in more detail.

Eosinophils are a type of white blood cell that belong to the granulocyte family. Granulocytes are named after the granules they contain in their cytoplasm, which store various substances that help the cells perform their functions. Eosinophils are characterized by their bi-lobed nucleus and their large, round granules that stain bright red-orange with eosin, a dye used in microscopy. Eosinophils make up about 1-6% of the total white blood cell count in healthy individuals, but their number can vary depending on the presence of infections, allergies, or other conditions.

Eosinophils are produced in the bone marrow from hematopoietic stem cells, under the influence of growth factors such as interleukin-5 (IL-5), granulocyte-macrophage colony-stimulating factor (GM-CSF), and interleukin-3 (IL-3). They circulate in the bloodstream for a few hours before migrating into various tissues, especially those exposed to the external environment, such as the skin, lungs, gastrointestinal tract, and genitourinary tract. Eosinophils can also be found in lymph nodes, spleen, thymus, and bone marrow. Eosinophils have a lifespan of about 8-12 days in tissues, but they can survive longer in certain inflammatory conditions.

Eosinophils are involved in several aspects of immunity and inflammation, mainly against parasitic infections and allergic reactions. They have receptors on their surface that allow them to recognize and bind to various molecules, such as immunoglobulins, complement components, cytokines, chemokines, and adhesion molecules. These receptors trigger different responses in eosinophils, such as activation, degranulation, phagocytosis, antigen presentation, cytokine production, and chemotaxis. Eosinophils can release the contents of their granules into the extracellular space or into phagocytic vacuoles. The granules contain a variety of proteins that have cytotoxic, pro-inflammatory, anti-inflammatory, or immunomodulatory effects. Some of these proteins include:

• Major basic protein (MBP): a cationic protein that can damage the membranes of parasites and host cells.
• Eosinophil cationic protein (ECP): a ribonuclease that can degrade RNA and DNA of parasites and host cells.
• Eosinophil-derived neurotoxin (EDN): another ribonuclease that can also inhibit viral replication and modulate nerve function.
• Eosinophil peroxidase (EPO): an enzyme that catalyzes the formation of reactive oxygen species (ROS) and hypobromous acid (HOBr) from hydrogen peroxide (H2O2) and bromide (Br-), which can kill parasites and host cells.
• Eosinophil-associated RNases (EARs): a family of ribonucleases that can degrade RNA and modulate immune responses.
• Charcot-Leyden crystals (CLCs): crystalline structures formed by the breakdown of eosinophil granules that can stimulate inflammation and tissue remodeling.

Eosinophils can also produce and secrete various cytokines, chemokines, growth factors, and lipid mediators that can regulate the recruitment, activation, differentiation, survival, or function of other immune cells, such as mast cells, basophils, T cells, B cells, macrophages, dendritic cells, fibroblasts, and epithelial cells. Some of these molecules include:

• Interleukin-4 (IL-4): a cytokine that promotes the differentiation of T helper 2 (Th2) cells and B cells into antibody-producing plasma cells.
• Interleukin-5 (IL-5): a cytokine that stimulates the production, maturation, activation

Eosinophils are granulocytes, meaning that they have granules in their cytoplasm that contain various substances that can be released upon stimulation. The granules of eosinophils are distinctive in their appearance and composition, and they are responsible for many of the functions and properties of these cells.

The size of eosinophils ranges from 10 to 16 micrometers in diameter, and they have a segmented or bi-lobed nucleus that occupies about 30% of the cell volume. The nucleus is connected by a thin strand of chromatin, giving the eosinophils a characteristic shape.

The cytoplasm of eosinophils is filled with two types of granules: specific and non-specific. The specific granules are larger and more numerous, and they have a unique crystalloid core that consists of four major cationic proteins: eosinophil peroxidase (EPO), eosinophil cationic protein (ECP), eosinophil-derived neurotoxin (EDN), and major basic protein (MBP). These proteins have antimicrobial, cytotoxic, and pro-inflammatory effects, and they can damage the membranes of parasites and host cells. The specific granules also contain other mediators, such as cytokines, chemokines, enzymes, and growth factors, that can modulate immune responses and tissue remodeling.

The non-specific granules are smaller and less abundant, and they contain lysosomal enzymes, such as cathepsins, acid phosphatase, and beta-glucuronidase. These enzymes can degrade extracellular matrix components and cellular debris.

The plasma membrane of eosinophils is studded with various receptors that allow them to sense and respond to different stimuli. Some of the receptors are specific for cytokines, such as interleukin-3 (IL-3), interleukin-5 (IL-5), and granulocyte-macrophage colony-stimulating factor (GM-CSF), that regulate the development, survival, activation, and migration of eosinophils. Other receptors are involved in adhesion and chemotaxis, such as integrins, selectins, and chemokine receptors. Eosinophils also express receptors for immunoglobulin E (IgE), complement components, toll-like receptors (TLRs), and Fc receptors, which enable them to participate in allergic and innate immune reactions.

The structure of eosinophils can change depending on their level of activation and the microenvironment they encounter. For example, activated eosinophils can undergo piecemeal degranulation, which is a process of selective release of granule contents through vesicles that fuse with the plasma membrane. This allows eosinophils to modulate their effector functions without causing excessive tissue damage. Activated eosinophils can also express more surface molecules, such as MHC class II proteins and co-stimulatory molecules, which enable them to act as antigen-presenting cells to T cells.

The structure of eosinophils reflects their multifaceted role in immunity and inflammation. By having a diverse array of granule contents and membrane receptors, eosinophils can respond to various stimuli and exert different effects on parasites, pathogens, host cells, and tissues.

Eosinophils are major effector cells in the immune system. They have a beneficial role in host defense against nematodes and other parasitic infections and are active participants in many immune responses. Eosinophils can also contribute to allergic inflammation, asthma, and other diseases.

Eosinophils possess a range of immunomodulatory factors that are released upon cell activation, including over 35 cytokines, growth factors, and chemokines. Cytokines are molecules that are used for cell signaling or cell-to-cell communication. Chemokines are a type of cytokine that attract other immune cells to the site of infection or inflammation. Growth factors are molecules that stimulate cell growth, differentiation, and survival.

Some of the cytokines produced by eosinophils include interleukin (IL)-2, IL-4, IL-5, IL-6, IL-10, IL-12, IL-13, IL-17, IL-18, IL-25, tumor necrosis factor (TNF)-α, and interferon (IFN)-γ. These cytokines can have various effects on other immune cells, such as promoting T cell proliferation and activation, inducing Th1 or Th2 polarization, enhancing B cell antibody production, activating mast cells and basophils, and regulating macrophage and dendritic cell functions.

Some of the chemokines produced by eosinophils include CCL5 (RANTES), CCL11 (eotaxin), CCL17 (TARC), CCL22 (MDC), CCL24 (eotaxin-2), CCL26 (eotaxin-3), CXCL8 (IL-8), CXCL9 (MIG), CXCL10 (IP-10), and CXCL16. These chemokines can recruit other eosinophils, as well as neutrophils, monocytes, lymphocytes, and natural killer cells to the site of infection or inflammation.

Some of the growth factors produced by eosinophils include granulocyte-macrophage colony-stimulating factor (GM-CSF), granulocyte colony-stimulating factor (G-CSF), vascular endothelial growth factor (VEGF), transforming growth factor (TGF)-β, platelet-derived growth factor (PDGF), and nerve growth factor (NGF). These growth factors can stimulate the proliferation and differentiation of hematopoietic stem cells, endothelial cells, fibroblasts, smooth muscle cells, and neurons.

Eosinophils can also release cytotoxic granule proteins that can kill unwanted cells or pathogens. These include major basic protein (MBP), eosinophil cationic protein (ECP), eosinophil-derived neurotoxin (EDN), and eosinophil peroxidase (EPO). These proteins can damage the membranes of parasites, bacteria, viruses, fungi, tumor cells, and host cells.

Besides producing cytokines, chemokines, growth factors, and cytotoxic proteins, eosinophils can also function as antigen-presenting cells. Antigens are anything that causes an immune response. Antigen-presenting cells can process and present antigens to T cells to activate them. Eosinophils can express major histocompatibility complex (MHC) class II proteins and co-stimulatory molecules that are required for antigen presentation. Eosinophils can also migrate to lymph nodes and present antigens to T cells there.

Eosinophils are involved in both innate and adaptive immunity. Innate immunity is the first line of defense against pathogens that does not require prior exposure or memory. Adaptive immunity is the second line of defense that is specific and long-lasting. Eosinophils can participate in both types of immunity by producing various mediators that modulate the immune response and by presenting antigens to T cells.

In summary, eosinophils are multifunctional cells that play an important role in immunity. They can defend against parasitic infections and other pathogens by releasing cytotoxic proteins. They can also regulate inflammation and tissue repair by releasing cytokines, chemokines, and growth factors. Furthermore, they can function as antigen-presenting cells and activate T cells. However, eosinophils can also be involved in pathological conditions such as allergic diseases and eosinophilic disorders.

Eosinophils are multifunctional cells that participate in various physiological and pathological processes. Some of the functions of eosinophils are:

• Host defense against parasites: Eosinophils are specialized in combating helminthic and protozoan infections by releasing cytotoxic granule proteins, reactive oxygen species, and lipid mediators that damage the parasite membrane and impair its viability. Eosinophils also secrete cytokines and chemokines that recruit and activate other immune cells to enhance the anti-parasitic response.
• Inflammation and allergy: Eosinophils are involved in the initiation and maintenance of inflammatory and allergic reactions, such as asthma, allergic rhinitis, atopic dermatitis, and eosinophilic esophagitis. Eosinophils release pro-inflammatory mediators, such as histamine, leukotrienes, prostaglandins, platelet-activating factor, and cytokines, that contribute to the symptoms of these conditions. Eosinophils also modulate the function of mast cells, basophils, T cells, B cells, dendritic cells, and epithelial cells through direct or indirect interactions.
• Antigen presentation: Eosinophils can act as antigen-presenting cells by expressing major histocompatibility complex (MHC) class II molecules and co-stimulatory molecules on their surface. Eosinophils can capture and process antigens from the environment or from parasites and present them to T cells in the lymph nodes or in the tissues. Eosinophils can also produce cytokines that influence the polarization and activation of T cells.
• Tissue remodeling and repair: Eosinophils can participate in tissue remodeling and repair after injury or infection by producing growth factors, extracellular matrix proteins, proteases, and anti-inflammatory mediators. Eosinophils can also modulate angiogenesis, fibrosis, and wound healing through their interactions with endothelial cells, fibroblasts, and other cell types.
• Homeostasis: Eosinophils can play a role in maintaining homeostasis in various tissues and organs by regulating the secretion of hormones, neurotransmitters, and neuropeptides. Eosinophils can also influence the metabolism of lipids, glucose, and iron through their granule proteins and cytokines.

Eosinophils are versatile cells that perform diverse functions depending on the context and stimuli. However, eosinophils can also cause tissue damage and dysfunction when they are dysregulated or overactivated. Therefore, understanding the biology and regulation of eosinophils is important for developing novel therapeutic strategies for eosinophil-associated diseases.

Eosinophils are a type of white blood cells that have diverse functions in the immune system and in various inflammatory and allergic conditions. They are characterized by their bi-lobed nucleus and their cytoplasmic granules that contain a variety of mediators that can modulate the immune response and cause tissue damage. Eosinophils are derived from hematopoietic stem cells in the bone marrow and circulate in the blood for a short time before migrating to different tissues. Eosinophils can be activated by various stimuli, such as cytokines, chemokines, allergens, and parasites, and can release their granule contents by degranulation or piecemeal degranulation. Eosinophils can also function as antigen-presenting cells and secrete cytokines that influence the polarization and activation of T cells. Eosinophils play a crucial role in host defense against parasitic infections, especially helminths, by releasing cytotoxic proteins and lipid mediators that damage the parasite membrane. However, eosinophils can also contribute to the pathogenesis of allergic diseases, such as asthma, allergic rhinitis, atopic dermatitis, and eosinophilic esophagitis, by causing inflammation, tissue remodeling, and airway hyperreactivity. Eosinophils can also be involved in other disorders, such as eosinophilia, hypereosinophilic syndrome, eosinophilic granulomatosis with polyangiitis, and eosinophilic leukemia. Therefore, eosinophils are important cells that have both beneficial and detrimental effects on human health.

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