An eosinophil is a type of white blood cell involved in immune system responses. These cells also appear to play a role in allergies and asthma when the body produces too many of them and they overreact to a perceived problem in the body, because they release substances which can be toxic. Like other types of blood cells, these cells can be counted when blood is drawn and separated, and an eosinophil count may be ordered to help a doctor understand what is causing a medical problem.
The eosinophil cell is named for a type of acidic dye. The ability to thrive in acidic environments is important for an eosinophil, as it allows the cell to reach many areas of the body. Eosinophils may also be referred to as acidophiles in some regions of the world, reflecting their acid-loving nature, although this terminology is not usual.
Normally, eosinophils make up around one to six percent of the white blood cells in the body. They are produced in the bone marrow, and they are part of a group of white blood cells known as granulocytes, because they are filled with small granules of material which they can release at the site of an infection or inflammatory reaction. Essentially, granulocytes are like mobile responders with their own tool kits which they can utilize to address an emerging medical problem quickly.
When an antigen is introduced to the human body, eosinophils are among the cells which arrive at the site to contain and neutralize the problem. An eosinophil can respond to bacterial, parasitic, and viral infections anywhere in the body, and the cells can also deal with ongoing inflammatory processes such as those caused by underlying infections or excessive immune responses. Typically, concentrations of these cells in the blood increase when someone is battling infection.
In a condition known as eosinophilia, there are too many of these cells in the blood. This can be a big problem, because when the cells degranulate, the compounds they carry can cause damage in high concentrations. When eosinophils flock to an area like the gut to respond to an infection and then flood the area with various compounds, they can cause irritation, inflammation, and other problems. In allergic responses, eosinophils overreact to an allergen, causing problems like skin irritation and difficulty breathing. Eosinpenia, in which there are too few, can be the result of steroid use or Cushing's Syndrome.