Yersinia enterocolitica is a bacterium capable of prolonged colonisation of the intestine without causing significant symptomatology. This bacterium is able to interact with the immune system, non-specifically enhancing cell-mediated immune responses. A consequence of this activity is the ability to induce resistance in the mouse to lethal doses of the intracellular pathogen Listeria monocytogenes. The study of non-specific interactions between Y. enterocolitica and immunity is of great interest because it provides insight into the mechanisms responsible for immunopathological manifestations (such as autoimmune thyroiditis) that are associated with human yersiniosis as aseptic sequelae. Likewise, the characterisation of bioactive fractions in this bacterium is the first step towards their possible application as immunopharmaceuticals.