There are large differences in the antigenicity and biology of the ReA-associated bacteria. For induction of arthritis, the relevance seems to be only that antigenic material reaches the joint, alive or dead. If there is a common antigen, it has to be a highly conserved one. Bacterial hsp60 seems to be an immunodominant T cell antigen in ReA, but there must be other relevant antigens shared by these different bacteria. An ineffective immune response (for example, low production of TNFalpha) seems to contribute to the manifestations and course of ReA. Although arthritis can also occur in its absence, HLA-B27 plays an important role in the pathogenesis of ReA and the other SpA. Current data suggest that B27 probably acts as an antigen-presenting molecule for a still-unknown arthritogenic molecule. Comparison of ReA with IBD-associated arthritis suggests that there might indeed be a common antigen shared by ReA-associated bacteria and bacteria of the gut flora. CD8+ T cells seem to be important in ReA and other SpA. In some parts of the world, such as in Mexico, ReA could be a major predisposing cause of the development of AS. Antibiotic treatment is not effective, probably because the triggering bacteria are already dead or in a partly latent state at the time arthritis occurs. Based on this knowledge and on new technologies, it should be possible in future years to derive answers to the questions about ReA and the other SpA and, as a consequence, to find a cure.