Does this mean that 256 tests need to be done to differentiate ME from controls or one molecule that has 256 parts in it?
I assume that they have 256 different peptide molecules, a tiny dab of each of which is put on to a different pin or well in an array.The patient's serum is then added and the pattern of antibody binding to the 256 pins (wells) is visualised using a fluorescent or otherwise tagged developer reagent. So each patient and control provides a pattern of 256 spots of different brightness. You put these into computer and it tells you whether it is a patient pattern or a control pattern - like a barcode really. What the text quoted above does not say is how well the pattern replicated across populations. With enough dots in the array you can always find a signature for patients, but it may be just a chance pattern in that sample.
One group of samples were from the earlier rituximab trials. So presumably one thing to watch out for will be another paper that applies the same protocol to generate immunosignatures from the post-treatment samples, to see if there are any interesting differences between the responders and non-responders and if the responders' immunosignatures look more like those of the healthy controls. Sample sizes are small of course.