Eosinophils play two roles in your immune system:

• Destroy foreign substances. Eosinophils consume matter flagged by your immune system as harmful. For example, they fight matter from parasites.
• Control infection. Eosinophils swarm an inflamed site when needed. This is important to fight disease. But too much can cause more discomfort or even tissue damage. For example, these cells play a key role in the symptoms of asthma and allergies, such as hay fever. Other immune system issues can lead to chronic inflammation as well.

Eosinophilia happens when eosinophils swarm a site in the body. Or when the bone marrow makes too many. This can happen due to many reasons including:

1. Parasitic and fungal diseases
2. Allergic reactions
3. Adrenal conditions
4. Skin disorders
5. Toxins
6. Autoimmune disorders
7. Endocrine conditions.
8. Tumors

Certain diseases and conditions that can cause blood or tissue eosinophilia include:

1. Acute myelogenous leukemia (AML)
2. Allergies
3. Ascariasis (a roundworm infection)
4. Asthma
5. Atopic dermatitis (eczema)
6. Cancer
7. Churg-Strauss syndrome
8. Crohn's disease — which causes tissues in the digestive tract to become inflamed.
9. Drug allergy
10. Eosinophilic esophagitis
11. Eosinophilic leukemia
12. Hay fever
13. Hodgkin lymphoma (Hodgkin disease)
14. Hypereosinophilic syndrome
15. Idiopathic hypereosinophilic syndrome (HES), an extremely high eosinophil count of unknown origin
16. Lymphatic filariasis (a parasitic infection)
17. Ovarian cancer — cancer that starts in the ovaries.
18. Parasitic infection
19. Primary immunodeficiency
20. Trichinosis (a roundworm infection)
21. Ulcerative colitis — a disease that causes ulcers and swelling called inflammation in the lining of the large intestine.

Parasites and allergies to medicines are common causes of eosinophilia. Hypereosinophilia can cause organ damage. This is called hypereosinophilic syndrome. The cause for this syndrome is often unknown. But it can result from some types of cancer such as bone marrow or lymph node cancer.