A method of diagnosing coeliac disease, or susceptibility to coeliac disease, in an individual comprising: (a) contacting a sample from the host with an agent selected from (i) the epitope comprising sequence which is: SEQ ID NO: 1 or 2, or an equivalent sequence from a naturally occurring homologue of the gliadin represented by SEQ ID NO: 3, (ii) an epitope comprising sequence comprising: SEQ ID NO: 1, or an equivalent sequence from a naturally occurring homologue of the gliadin represented by SEQ ID NO: 3, which epitope is an isolated oligopeptide derived from a gliadin protein, (ii) an analogue of (i) or (ii) which is capable of being recognised by a T cell receptor that recognises (i) or (ii), which in the case of a peptide analogue is not more than 50 amino acids in length, or (iv) a product comprising two or more agents as defined in (i), (ii) or (iii), and (b) determining in vitro whether T cells in the sample recognises the agent; recognition by the T cells indicating that the individual has, or is susceptible to, coeliac disease. Therapeutic compositions which comprise the epitope and gliadin proteins which do not cause coeliac disease are also provided.