Eosinophils are a type of leukocytes and their high ratio can be seen in: 1) allergic diseases such as allergic asthma, allergic dermatitis, etc.; 2) parasitic infections such as Ascaris lumbricoides, liver fluke, etc. can lead to high reactive eosinophils; 3) infectious diseases, some patients with fungal infections and tuberculosis infections have high peripheral blood eosinophils; 4) various skin diseases can also lead to high eosinophils such as Eczema, herpetic dermatitis, psoriasis, etc.; 5. Some autoimmune diseases such as lupus erythematosus, rheumatoid arthritis, etc. All of these are common causes of reactive eosinophilia, that is, eosinophilia secondary to other diseases. There is also a group of causes due to increased bone marrow production of eosinophils, that is, primary causes, mainly hematopoietic diseases, such as the relatively rare eosinophilic leukemia and chronic granulocytic leukemia.