What are Eosinophils?

Eosinophils Explained

First discovered in 1879, eosinophils are white blood cells identified in laboratory studies by their propensity to absorb the red dye, Eosin. They are one of five types of white blood cells in the body that form the basis of a healthy immune system and are most often related to parasitic infection and allergic response. Though scientists have known about these cells for over 130 years, the majority of information available today has emerged since 1990 when new laboratory techniques made it possible to utilize small sample sizes for greater experimentation. This enhanced research capability has enabled greater understanding of eosinophils and their role in immune recovery and allergic disease.

Cell Structure and Circulation Behavior

Normally, eosinophils are present in very low levels in the body, comprising 0-3% of all active white blood cells circulating at one time and are found in the blood stream, esophagus, intestines, stomach, heart, lungs, and skin. These granule-filled cells usually contain a two-lobed nucleus though some illnesses alter the cell structure and increase the nucleus count to four. The granules themselves occupy 20% of the cell’s cytoplasm and contain a number of enzymes and toxic proteins used to fight infections and foreign entities.

The production and distribution of eosinophils throughout the body is controlled by the creation and release of cytokene proteins from other cells. They are reactive and mature quickly in the bone marrow for release into the circulatory system; however, they do not reside in the blood stream for long. Approximately half of the active cells will migrate from the blood to other tissues in the body on the first trip through the circulatory system. Fast acting transference from blood to tissue ensures fast response capability when foreign entities are identified.

Healthy Immune Response

All five of the white blood cells in the immune system work together in specialized roles to increase disease-fighting effectiveness. Eosinophils are a critical part of this immune collaboration specially designed to overwhelm and destroy parasitic infections. Each cell contains multiple forms of enzymes and inflammatory proteins. When a parasite invades healthy tissue, the chemistry of the tissue changes and releases a signal for help. This signal attracts the toxin-filled eosinophils and they move to the location of the system’s invader. They attack by engulfing the parasite, or in the case of larger parasites like worm variants, they attach to the intruder and degranulate, secreting their enzyme and protein combinations resulting in the digestion of the offending organism.

Role in Allergic Response

In addition to parasite destruction, eosinophils help regulate and control allergic reactions. Unfortunately, the immune system is not perfect and allergic responses are the result of a misinterpretation. When a harmless substance, like tree pollen, is perceived as a dangerous and potentially damaging invader the body responds with histamine to intercept and prevent illness. In severe reactions, mast cells release a chemical known as arylsulphatase which is responsible for anaphylaxis, the most severe and life threatening type of allergic response. Eosinophils can counteract both histamine and arylsulphatase decreasing the likelihood of severe response and creating an essential balance in the system.

Yet, these white blood cells do not always deliver positive results in allergic response. As tissues mis-identify allergens as parasites, allergy sufferers begin to experience increased eosinophils levels. Though effective in the destruction of parasites, the enzymes and proteins in these cell granules also damage normal healthy body tissue. The ongoing damage and inflammation to organs and tissues exposed to excessive secretions shifts these disease-fighting cells from the cure of illness to the cause of allergic disease. In the most severe cases, this can escalate into a full Eosinophilic Disorder.

Eosinophilic Disorders (EGID)

Chronic illnesses caused by high levels of eosinophil accumulation in organs and their surrounding tissues, EGIDs are associated with ongoing and often extreme allergic responses. Both children and adults are susceptible and these disorders are difficult to diagnose and have no cure. Three of the common EGIDs impact areas of the digestive system and are most often attributed to severe food allergies:

  • Eosinophilic Colitis is caused by cell accumulation in the large intestine and is frequently misdiagnosed as Irritable Bowel Syndrome. Symptoms include nausea, with or without vomiting, diarrhea, abdominal pain, and bloody stool.
  • Eosinophilic Esophagitis creates inflammation of the esophagus resulting in difficulty swallowing and damage to the muscles of the esophagus. There has been an increase in the awareness and research of this condition over the past 10 years so more and more people are being diagnosed.
  • Eosinophilic Gastroenteritis and Gastritis results from excessive cell infiltration in the small intestine and stomach. It most commonly presents as severe abdominal pain with vomiting and diarrhea.
The most common treatments for accumulated eosinophils in the digestive systems include dietary exclusions to avoid food allergens, steroid therapies, and when needed, feeding tubes to sustain adequate nutritional levels. Even with treatment consistent monitoring of cell levels is required to ensure more serious complications do not arise.

There are still serious EGIDs that have not yet been associated with an underlying cause. These diseases, such as Churg-Strauss Syndrome and Hypereosinophilic Syndrome, are less common than food allergy-based disorders but have more severe symptoms and disease progression. It is possible they are related to allergy responses that have yet to be identified.

EGIDs are difficult for healthcare professionals to diagnose as there is no simple test to identify excess eosinophils. Blood tests can indicate elevated levels within the circulatory system, but this is not necessarily an indicator of abnormal illness. The only current method available to diagnose new EGID illnesses, or monitor previously identified cases, is through tissue biopsies of the inflamed areas. The invasive nature of this diagnostic method increases the reluctance of both physician and patient which can increase the time needed to obtain the appropriate treatment. Patients ultimately diagnosed with an EGID often have additional allergic diseases such as hay fever, allergic rhinitis, asthma, and atopic dermatitis. Information about other allergy-related illnesses can be an important indication that early testing is warranted. Left undiagnosed Eosinophilic Disorders damage the body, impact quality of life, and in rare cases can prove fatal.

Eosinophils are important cells that fight off deadly parasites, prevent anaphylactic reactions to allergens, and maintain health. Yet, they are also responsible for illness in some increasing the chance of serious inflammation and tissue damage. Though an important component in the body designed to fight off illness, these cells can have negative health consequences as well and are known to play a role in many diseases including several chronic autoimmune conditions.

For more detailed information check out the list of topics about eosinophilic esophagitis.

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