Eosinophils are a type of white blood cell found mainly in the mucosal lining of the respiratory and gastrointestinal tracts and in the adjacent skin, differentiated from bone marrow stem cells, and closely related to the immune system. Eosinophils have a weak phagocytic effect and play a role in limiting allergic reactions. In normal individuals, eosinophils are low during the day and high at night, but they fluctuate within the normal range. If eosinophils exceed the normal range and are in a high state, it may be caused by the following diseases: 1. Allergic diseases: such as urticaria, allergic asthma, allergic purpura, etc. In the case of urticaria, it can be accompanied by wind clumps as well as intense itching. In case of allergic asthma, sneezing, runny nose, wheezing, shortness of breath as well as difficulty in breathing can be seen. Typical symptoms of allergic purpura are purplish papules on the skin and some patients may experience abdominal pain and swollen joints. In addition, drug allergy may also lead to high eosinophils; 2, parasitic infections: such as roundworm, hookworm, pinworm, schistosome, tapeworm, toxoplasma and other infections may lead to high eosinophils; 3, other diseases: such as dermatitis, psoriasis and other skin diseases, Hodgkin’s disease, pernicious anemia, multiple myeloma and other hematologic diseases, ulcerative colitis and other gastrointestinal diseases, nodular poly Eosinophilia may occur in connective tissue diseases such as arteritis and rheumatoid arthritis, endocrine diseases such as hypopituitarism and Addison’s disease, and tumor spread. In addition, eosinophilia can be reduced in acute infectious diseases, but scarlet fever can cause eosinophilia to be high. If eosinophilia is high, it is necessary to combine bone marrow imaging, immunological examination, and biopsy to identify the cause of the disease and to provide different treatments, such as anti-allergic treatment, anti-parasitic treatment, and anti-infective treatment, for each cause.