The positive effect of interferon as a therapy for multiple sclerosis was mentioned in a previous post. Its performance is judged by its reduction of the number of brain lesions seen on MRT scans. This is statistical data and the individual patient may ponder the question of what these statistics mean for her or his own case. The way in which interferon combats MS is not well understood. New insights on this question are presented in a recent paper of Prinz et. al. (Immunity 28, 675). My attention was drawn to this work by a talk given by one of the authors, Ulrich Kalinke, in Berlin on 15th May. He said little about EAE but the talk encouraged me to look further. An account of this work which is less technical than in the original paper can be found in a paper of Axtell and Steinman (Immunity 28, 600) This work concerns the model disease EAE in mice. It is found that in mice with the disease interferon is produced in the central nervous system but much less in other tissues. So it seems that the body itself is producing the interferon as a defence against inflammation of the central nervous system. This substance is known to have effects on many immune cells and it is a priori not clear which of these effects are relevant for disease progression in EAE. Prinz. et. al. find that the effects of the interferon on certain cell types (T-cells, B-cells and astrocytes) seem to be irrelevant while the important thing is its effect on microglia and macrophages. This could be an important step towards pinpointing the mechanism leading to the therapeutic effect of interferon .