OSINOPHIL production can be induced in vitro by E interleukin-3 (IL-3), granulocyte-macrophage colonystimulating factor (GM-CSF), and IL-5. Both IL-3 and GM-CSF have activities on other hemopoietic lineages, whereas IL-5 is specific for the eosinophil/basophil lineage. The activity of IL-5 on basophils has been reviewed recently'and will not be discussed here. There is now good evidence that IL-5 is the major, and possibly the only, cytokine involved in the production of specific eosinophilia. Classically eosinophilia is observed in a restricted number of diseases, most notably helminth infections and allergic diseases. However, although the increase in eosinophil numbers is less spectacular, there is increasing evidence for an involvement of eosinophils in a wider spectrum of diseases.

A study of experimental infection of volunteers by the hookworm Necator americanus gave clear evidence for an increase in eosinophils with no significant increase in other blood leucocytes, 2 and infection of mice with the cestode Mesocestoides Corti causes a massive increase in blood and peritoneal eosinophils with little effect on the number of ne~ trophils.~ This biologic specificity, the relatively restricted disease profile of eosinophilia, and the requirement for a functional T-lymphocyte~ ystem~-~ suggested that the control of eosinophilia must be part of the immune response and based on a subset of T celk7 Eosinophils are cytotoxic cells capable of killing helminths, parasitic protozoa, chicken erythrocytes, and tumor cells in vitro. For more general reviews and background see Spry, 8 Dexter et a1, 9 Thompson,'O and Smith and Cook. ll Time-lapse cinematography has shown that eosinophils phagocytose and regurgitate red blood cells very rapidly. This is in contrast to neutrophils and macrophages that retain them until they have been broken down. 12 This explains why eosinophils appear to be less active in comparison to neutrophils in classical phagocytosis assays using yeast or bacteria. They do not retain the particles. It is unclear what this implies for the killing ability of the eosinophil, but it is possible that they are better adapted to attach to large parasites13 rather than to phagocytose small particles.