Eosinophil granulocyte

Humans have an innate defense mechanism that first responds to signs of infection or toxicity, as well as a specific immune defense, which develops in response to a specific type of microbe or foreign substance. It has been shown that there is more than one line of immune defense. White blood cells, of which there are several types, are at the foreground of the innate immune response. Eosinophils are a type of white blood cell that plays a vital role in defense against infection and inflammation.

Understanding the role of human immunity is a daunting task, as there are a great number of different immune components, and each of these components often has more than one role in the immune response. Throughout history, technological advances have enabled researchers to characterize and investigate the different factors of the immune system, and though much has been learned, much more has yet to be discovered.

Background

Eosinophils, a type of granulocyte (white blood cell), was first discovered in 1846 but was not characterized until Paul Ehrlich demonstrated that the cell stained red with aniline (eosin) dyes in 1879. Granulocytes are a type of white blood cell that is tasked with destroying foreign substances in the blood, mainly those that cause toxicity, infection, or allergies. Eosinophils possess a number of strongly basic proteins in their granules that allow them to bind effectively with acidic eosin dyes.

These cells are mostly known for their role in attacking parasitic infections and the modulation of Type I hypersensitivity reactions (allergies). However, they also plays a regulatory role in inflammation. The prolonged presence of eosinophils in the blood can exacerbate tissue destruction and is largely responsible for airway remodeling in chronic asthma patients, where inflammation is constantly present. The same holds true for individuals with autoimmune diseases such as Lupus, rheumatoid arthritis, exfoliative dermatitis, and cancer.

Innate, or nonspecific, immunity is the first line of defense in the body from invasion of foreign substances. This type of immunity is found in skin, tears, mucus, saliva and is deployed to areas of inflammation by specific cell signaling produced after injury or infection. As a first line defense, an innate immune mechanism can deter the entrance or spread of infection, but it is not adequate to prevent the infection completely. After the innate immune response, a cascade of immune response follows, according to the type of infection or foreign substance that is present in the body. This is called the adaptive or specific immune response. The specific immune response is responsible for the development of antibodies against a particular substance, such as bacteria or viruses. These microbes have factors on their cell or capsid surfaces that allow the specific immune system to mount a defense if the body is exposed again. In most cases, this is sufficient to prevent a reinfection.

Overview

The eosinophil granulocyte is a type of white blood cell that is derived from progenitor cells during hematopoiesis (blood formation) in the bone marrow. They are a part of the innate immune system and serve several functions, from inflammation to defense against parasites and viral infections. They are phagocytic but are less efficient in destroying intracellular bacteria than neutrophils. Eosinophils can modulate immediate hypersensitivity reactions (allergies and asthma) by inhibiting mediators released by mast cells (histamine and leukotrienes), lysophospholipids, and heparin. Prolonged exposure to high levels of eosinophils can cause tissue damage, though exact mechanisms are poorly understood.

Eosinophils comprise about 1% to 6% of white blood cells. They reside in brain tissue (medulla), bowel, the female reproductive system, and lymph tissue (e.g., spleen and lymph nodes). If eosinophils are found in other tissue, such as the lungs, epithelial, or upper gastrointestinal tissue, it can suggest a disease process is present. Once eosinophils are released into the blood stream, they persist for approximately eight to twelve hours and can survive for eight to twelve days in targeted tissues.

Eosinophils develop from myeloid precursor cells located in bone marrow, in the presence of cytokines, granulocyte macrophage–colony stimulating factor, multiple types of growth factors, leukotrienes and more factors. These factors are released in response to a specific type of immune attack. After the eosinophils are released to inflammatory sites they can produce granule proteins, reactive oxygen species (peroxidase), eicosanoids and prostaglandins (produceing pain and cramping), enzymes (i.e., elastase), growth factors, and cytokines. It has also been observed that eosinophils can present antigen to T cells.

Granule proteins are produced in response to an immune stimulus, which are cytotoxic and can result in tissue damage. The major eosinophilic protein is eosinophil peroxidase, which is quite toxic to cells. It achieves this by creating toxic pores in target cells that usher additional cytotoxic molecules into the cell. The peroxidase produces reactive oxygen species (free radicals) that result in oxidative stress in the target cell, causing cell death by apoptosis or necrosis.

A serious condition can develop when eosinophils are greater than 500/µL of blood, called eosinophilia. This condition can develop in patients with parasitic diseases, rheumatoid arthritis, Hodgkin disease, exfolative dermatitis, Addison disease, and reflux esophagitis. It also occurs in patients with severe asthma. Eosinophilia is routinely treated with anti-inflammatory medications, such as corticosteroids, monoclonal antibody therapy, and mast cell inhibitors.

In summary, as with any white blood cell, eosinophils have the very important role of defending the body against infection or inflammation. It can facilitate the destruction of foreign substances or microbes by mechanical (phagocytosis) or chemical (immune factor modulation) means. However, prolonged exposure of this type of granulocyte can cause vast tissue damage and can prolong recovery of infection if not promptly and correctly treated.

Bibliography

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