As will be discussed here, reproductive immunology is a very good example of how paradigms have shaped our understanding of immune regulation but don’t provide all of the answers. A central paradigm of modern

immunology is the clonal-selection theory, formulated by F. MacFarlane Burnet1 in the late 1950s, which explains how immune system makes antibody responses to diverse antigens and IWR-1 nmr discriminates self from non-self. The key features of the clonal-selection theory are that (i) each lymphocyte bears antigenic receptors of a single specificity; (ii) receptor specificity and diversity is germline-encoded, randomly generated and precedes antigen encounter; (iii) lymphocytes with receptors that recognize self-molecules are deleted at an early stage of development; and (iv) antigen encounter of mature lymphocytes leads to clonal expansion and consequently adaptive immunological memory. The clonal-selection theory has prompted

much debate and been Hydroxychloroquine supplier challenged as being over-simplified in its view of self–non-self discrimination by (among others) Polly Matzinger’s Danger model and Charles Janeway’s pathogenicity model.2 However, it is worth noting that Burnet made his discovery in an era prior to the development of all the transgenic and knock-out mice, molecular probes and monoclonal antibodies (moAbs) that now permit a more detailed dissection of the immune system and test the predictions of paradigms more fully. MacFarlane Burnet’s work was groundbreaking, and he shared the 1960 Nobel Prize for Medicine or Physiology with Peter Medawar for the discovery of immunological tolerance (http://nobelprize.org/nobel_prizes/medicine/laureates/). However, Peter Medawar was also among the first to recognize that a simple self–non-self model was not absolute in its predictions of immunological tolerance and immune activation, as it could not explain the phenomenon of mammalian

pregnancy Histamine H2 receptor in the face of a functional maternal immune system. Medawar3 formulated three hypotheses that could help explain placentation and mammalian reproduction within the context of self–non-self discrimination. These hypotheses formed the basis of three new paradigms of reproductive immunology, namely that (i) the maternal immune system is suppressed; (ii) the placenta acts a barrier between the mother and foetus; and (iii) the foetus is antigenically immature and therefore not recognized by the maternal immune system. The status of these paradigms was eloquently reviewed by David Billington4 in 2003 to mark the 50th anniversary of Medawar’s publication. With better immunological tools, we now know that Medawar’s paradigms were over-simplified, with the exception of the importance of anatomical separation of the mother and foetus by the placenta. However, like other important paradigms, they fuelled key discoveries in reproductive immunology and in turn have led to the formulation of modified and new paradigms.