Understanding Eosinophils: The White Blood Cell Quiet Giants

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Blood cells in our body are divided into red cells, white cells and platelets. The white blood cells (WBCs) are divided into granulocytes (they have granules with different substances in their membranes that are visible microscopically), and non-granulocytes (the most common cells in this class are lymphocytes, which produce antibodies and are important in long term body defense against certain invaders).

The granulocytes are further divided into three categories. Neutrophils are the most common white blood cells making up 50 to 60 percent of all WBCs. Also present are basophils, which are visible microscopically when exposed to an alkaline stain. And finally, eosinophils are visible with an acidic stain (combination of Eosin and methylene blue). Clearly you can see how eosinophils get their name.

A quick backstory on eosinophils

Eosinophils are very easy to spot on the microscope because they have the largest granules that stain red. They compose 6 percent or less of the whole white cell count. While most of the white cells circulate in the blood, eosinophils mostly reside in tissue. Debate continues about what exactly is the significance of elevated levels of eosinophils in the blood.

What do eosinophils do?

Some theories suggest that the elevated eosinophil count is either a cause of disease, a consequence the disease, or just an innocent bystander phenomenon. I would agree that the most accurate explanation is probably that it plays a different role depending on the type of disease.

When elevated levels of eosinophils in the blood circulate this may represent:

  • They are increasing and mobilizing to fight parasites or bacteria
  • They are increasing and mobilizing to kill “invader” cells
  • They are increasing to participate in allergic reactions
  • A part they play part in food and medication allergies
  • A part they play in different types of pneumonias that involves the inflammatory mediators that derive from the eosinophils.

Diseases marked by eosinophilia

There are a variety of diseases that are characterized by elevation in the counts of eosinophils in the blood and they are known collectively as Hypereosinophilic Syndrome (HES). There are also other types of pneumonia accompanied by elevation of eosinophils which are known collectively as Pulmonary infiltration with eosinophilia (PIE syndrome).

The diseases known as the HES are the ones where the eosinophils increase and spread, causing direct organ damage, usually through the chemicals and mediators contained in the granules of the cells that get released.

The most common organs that are involved and have the potential to be damaged are the lungs and the gastrointestinal tract.

One example of HES eosinophilic disease is a resistant form of asthma that also involves the blood vessels called [Eosinophilic granulomatosis with polyangitis (EGPA)](https://www.hopkinsvasculitis.org/types-vasculitis/churgstrauss-syndrome-css/ syndrome), also known as Churg-Strauss. In this case there are eosinophils seen in the biopsies of blood vessels. There’s essentially an excessive number of eosinophils in the blood, fever, and accompanying vasculitis of various organ systems.

Other conditions with an eosinophilic component

One type of pneumonia called eosinophilic pneumonia starts with an infection and is then perpetuated by the action of the eosinophils.

One type of infection that can occur in the lungs is the one caused by roundworm (ascaris) which has a lifecycle that allows the eggs to reach the lung through the blood stream. There the invader proliferates and the consequences are proliferation of eosinophils. This is a condition called Loffler Syndrome.

Another lung condition that is characterized by elevation of eosinophils is a type of asthma that is an allergic reaction to a fungus that can invade the lung called aspergillus. This condition is known as Allergic Bronchopulmonary Aspergillosis (ABPA). This type of asthma is different from the traditional trajectory of asthma in that it causes more destruction of the bronchial airways.

Since eosinophils are produced in the bone marrow, that is also a potential site for disease. An abnormal clone of cells of this type can grow uncontrollably and is one type of cancer-like process is called Acute Eosinophilic Leukemia.

How do you treat eosinophil-driven diseases?

The problem with treating some of these infections is that not only antibiotics are needed to kill the infectious agents, because as these agents are killed, they release mediators that prolong the inflammation. At this point, anti-inflammatory medications (usually corticosteroids) are necessary to stop the secondary process. This secondary treatment, in turn, may lower the body’s immune defenses, risking worsening the infection. It can into a dangerous cyclical disease process. One very large risk in this prolonged process is permanent destruction of tissue.

The eosinophils are not only active in infectious processes, but they also act as signals that indicate an allergic process. Since the eosinophil resides mostly in subcutaneous tissue rather than the blood, it is typically active wherever the allergic process starts in the tissue. As an allergic process causes swelling from the leakage of fluids (think weeping hives), the eosinophils become more active. One theory is that the eosinophil proliferation in this case is an effort to contain the damage.

Why eosinophils are important

Clearly, the eosinophil is actually a very remarkable cell. It’s an agent in propagating inflammatory conditions, or levels can increase in response to other mediators in order to help control and limit allergic reactions.

The association of eosinophils with so many different disease processes offers the medical community the potential to identify clues that may help to fuel research in order to discover how eosinophils can be manipulated to control disease outcome.