Eosinophils are a type of white blood cell that play a key part in the body's immune system. They are responsible for fighting off invasions from other organisms, such as parasites and viruses. Eosinophils are made in the body's bone marrow, and then circulate around the body via the blood. During a blood test, health care providers check for eosinophil levels. High levels are a warning sign that the body's immune system is quite active. Low eosinophil levels are preferred, according to Net Wellness.

Successful Control of Allergies

Allergic or atopic disorders trigger eosinophils to increase throughout the body. For example, when someone is affected by hay fever, there are increased levels of eosinophils in the nasal passages, notes CincinnatiChildrens.org. When a person's allergic condition is well-managed, these white blood cell levels are low. Over-the-counter or prescription antihistamines can control the body's reaction to allergens, thereby signaling the body that the production of eosinophils in the bone marrow are not necessary, according to CincinnatiChildrens.org.

Proper Asthma Management

When a patient experiences an asthma attack, the eosinophils are not properly released and accumulate in the lungs. They release toxins to combat intruders, but instead inflame the tissues of the lungs. Steroid medication is generally successful in eliminating this process, so when a patient's asthma is being managed properly via rescue inhaler and other medications, the eosinophil count is low, notes Global Net. Managing a patient's asthma can also eliminate long-term damage to the lungs from an overabundance of eosinophil toxins.

Absence of Conditions

Because low levels of eosinophils means that the body is not likely to be engaged in fighting off illnesses, a blood test may help rule out several conditions, including certain drug reactions, immunologic disorders, blood disorders, endocrine problems, parasitic infestations and infectious diseases, according to Merck. When high concentrations of eosinophil is discovered in tissue, such as the nasal passages, it can give heath care workers a clue about what could be wrong. When eosinophil levels are found to be elevated, it gives health care workers more clues about what might be causing varying levels.