Eosinophils are immune response cells located in the peripheral blood, bone marrow, and lymph nodes, among others; an increase in the number of eosinophils in the peripheral blood above 5000/mm 3 is associated with conditions ranging from infections (bacterial and parasitic) and allergy (asthma, rhinitis, or drugs), even neoplasms. Various study groups have classified them according to their etiology, thus facilitating their diagnosis and treatment. The WHO divides them as primary and secondary and also considers the number of eosinophils/mm 3 and the involvement of white organs, while others have divided them into intrinsic and extrinsic. The former include mutations in the pluripotential hematopoietic cells, which lead to chronic myeloid leukemias with clonal expansion of eosinophils and extrinsic ones where the changes are related to a TH2 response activated by different cytosines such as IL-5. Current treatments are specifically aimed at modifying the clonal expansion of eosinophils with corticosteroids, hydroxyurea, interferon (peg) alpha, imatinib, among others, and bone marrow transplantation, while in extrinsic alterations corticosteroids and IL inhibitors are used −5 (mepolizumab).