Eosinophilia (e-o-sin-o-FILL-e-uh) is a higher than normal level of eosinophils. Eosinophils are a type of disease-fighting white blood cell. This condition most often indicates a parasitic infection, an allergic reaction or cancer.

You can have high levels of eosinophils in your blood (blood eosinophilia) or in tissues at the site of an infection or inflammation (tissue eosinophilia).

Tissue eosinophilia may be found in samples taken during an exploratory procedure or in samples of certain fluids, such as mucus released from nasal tissues. If you have tissue eosinophilia, the level of eosinophils in your bloodstream is likely normal.

Blood eosinophilia may be detected with a blood test, usually as part of a complete blood count. A count of more than 500 eosinophils per microliter of blood is generally considered eosinophilia in adults. A count of more than 1,500 eosinophils per microliter of blood that lasts for several months is called hypereosinophilia.

Eosinophils play two roles in your immune system:

• Destroying foreign substances. Eosinophils can consume foreign substances. For example, they fight substances related to parasitic infection that have been flagged for destruction by your immune system.
• Regulating inflammation. Eosinophils help promote inflammation, which plays a beneficial role in isolating and controlling a disease site. But sometimes inflammation may be greater than is necessary, which can lead to troublesome symptoms or even tissue damage. For example, eosinophils play a key role in the symptoms of asthma and allergies, such as hay fever. Other immune system disorders also can contribute to ongoing (chronic) inflammation.

Eosinophilia occurs when a large number of eosinophils are recruited to a specific site in your body or when the bone marrow produces too many eosinophils. This can be caused by a variety of factors, including:

• Parasitic and fungal diseases
• Allergic reactions
• Adrenal conditions
• Skin disorders
• Toxins
• Autoimmune disorders
• Endocrine disorders
• Tumors

Specific diseases and conditions that can result in blood or tissue eosinophilia include:

• Acute myelogenous leukemia (AML)
• Allergies
• Ascariasis (a roundworm infection)
• Asthma
• Atopic dermatitis (eczema)
• Cancer
• Churg-Strauss syndrome
• Crohn's disease (a type of inflammatory bowel disease)
• Drug allergy
• Eosinophilic esophagitis
• Eosinophilic leukemia
• Hay fever (allergic rhinitis)
• Hodgkin's lymphoma (Hodgkin's disease)
• Hypereosinophilic syndrome
• Idiopathic hypereosinophilic syndrome (HES), an extremely high eosinophil count of unknown origin
• Lymphatic filariasis (a parasitic infection)
• Ovarian cancer
• Parasitic infection
• Primary immunodeficiency
• Trichinosis (a roundworm infection)
• Ulcerative colitis (a type of inflammatory bowel disease)

Parasitic diseases and allergic reactions to medication are among the more common causes of eosinophilia. Hypereosinophila that causes organ damage is called hypereosinophilic syndrome. This syndrome tends to have an unknown cause or results from certain types of cancer, such as bone marrow or lymph node cancer.

Eosinophilia is usually found when your doctor has ordered blood tests to help diagnose a condition you're already experiencing. It's usually not an unexpected finding, but it's possible that it may be discovered simply by chance.

Talk to your doctor about what these results mean. Evidence of blood or tissue eosinophilia and results from other tests may indicate the cause of your illness. Your doctor may suggest other tests to check your condition.

It's important to determine what other conditions or disorders you may have. If you get an accurate diagnosis and can receive treatment for any relevant conditions or disorders, the eosinophilia will likely resolve.

If you have hypereosinophilic syndrome, your doctor may prescribe medications, such as corticosteroids. And he or she will want to monitor your health, as this condition may cause significant complications over time.