The present invention relates to improvements in therapy and prevention of conditions characterized by an elevated level of eosinophil leukocytes, i.e. conditions such as asthma and other chronic allergic diseases. A method is provided for down-regulating interleukin 5 (IL5) by enabling the production of antibodies against IL5 thereby reducing the level of activity of eosinophils. The invention also provides for methods of producing modified IL5 useful in this method as well as for the modified IL5 as such. Also encompassed by the present invention are nucleic acid fragments encoding modified IL5 as well as vectors incorporating these nucleic acid fragments and host cells and cell lines transformed therewith. The invention also provides for a method for the identification of IL5 analogues which are useful in the method of the invention as well as for compositions comprising modified IL5 or comprising nucleic acids encoding the IL5 analogues. The preferred embodiment of the present invention entails the use of variants of IL5, where foreign T helper epitopes are introduced so as to induce production of cross-reactive antibodies capable of binding to autologous IL5.