The immunoregulatory pathways are based on populations of lymphocytes, termed regulatory T cells (T reg s), in which there is currently considerable interest. In the case of infectious disease, such populations may lead to rapid resolution, the establishment of latent or persistent infection or to tissue damage by autoimmune processes [28] . Accordingly, T reg s have been termed 'a dangerous necessity' [29] . This term implies that T reg s are neither 'good' or 'bad' per se but may, according to the overall pattern of responsiveness, participate in appropriate immune reactions leading to resolution of disease or in inappropriate ones resulting in immunopathology.