ABSTRACT
Until this point, our discussion of the adaptive immune response has centred on the activation and amplification of effector T and B lymphocytes and the various positive signals that must be delivered to induce such reactions (positive regulation or up-regulation). This chapter examines the opposite immunological phenomenon, that of switching off (suppressing or down-regulating) an adaptive immune response when it is no longer required. Once the immune system has eliminated foreign antigen, it has no need to maintain the large population of effector cells and molecules. In fact, if such cells persisted in the absence of antigen, there would be a risk that they might turn upon normal tissue and induce secondary immunopathology. Therefore, the immune system must have evolved strategies to detect the elimination of antigen and to suppress effector cell function. At the same time, it is crucial that the memory lymphocyte populations are retained in case of secondary antigenic exposure.
