There are provided peptides derived from antibodies with reactivity against a GPI linkage epitope and functionally-equivalent ligands. These peptides can be used in the therapy and diagnosis of a variety of diseases, all of which are considered to be caused by the inappropriate presence in the body of autoantibodies which are reactive with GPI linkage epitopes. There is also described a mechanism of action of these autoantibodies which compromises the organism, so causing disease, and a method of prevention of disease and detection of the autoantibody.