Polynuclear eosinophils play a major role in host defense against infectious diseases and especially helminthiasis. Onset of hypereosinophilia can be attributed to two mechanisms which can occur separately or in combination. The first mechanism involves enhancement of medullary production and differentiation of eosinophils. The second is prolonged life span of eosinophils. Response to eosinophils depends on various cytokines including IL-5, IL-4, IL-3 and GM-CSF. Since it contains the genes coding for these cytokines, the 5q31-q33 region of chromosome 5 is the focus of study on genetic control of human hypereosinophilia. The goal of these studies is to allow screening of subjects predisposed to helminthic infection and to deregulation of immune responses that may lead to atopy and various types systemic inflammatory diseases.