Immune privilege: a recurrent theme in immunoregulation?

Abstract

The word ‘privilege’ implies a special status. The concept of immune privilege was coined in the mid-20th century to describe observations that immunogenic tissues and tumors placed at certain anatomical locations survived longer than anticipated or were accepted indefinitely. The implication was that these anatomical sites were in someway specialized because otherwise potent antigenic stimuli failed to elicit effective immune responses. Historically, passive mechanisms, notably physical and physiologic barriers that maintained local segregation between tissues and immune cells and antibodies, were thought to offer the most likely explanations for localized immune privilege. However, the recognition that multiple mechanisms exist to suppress and subvert adaptive immunity and promote tolerance raises the prospect that active suppressive mechanisms help to maintain immune privilege. This volume of Immunological Reviews was inspired by the original concept of immune privilege and by recent rapid progress in identifying immunoregulatory processes that suppress immunity and promote immune tolerance. Our goal in this issue is to assess contemporary thinking on classical immune privilege and to determine if the concept of immune privilege might have wider implications and value for understanding immunoregulation in general. To this end, we solicited reviews from experts working on traditional immuneprivileged tissues and on immunoregulatory processes. The classical definition of immune privilege and the consequences of active immunoregulation overlap in the sense that these processes prevent elaboration of effective immunity following challengeswith otherwise potent antigenic stimuli. In each case, antigenic challenge provokes weak and ineffective responses,