Pregnancy, a test case for immunology

The traditional conception of immune function is that of a system which differentiates the organism’s own tissues (the self) from any foreign invaders (nonself), preserving the former by rejecting the latter. In a mammalian pregnancy, however, the immunologically foreign foetus is accepted by the gestator’s immune system. This presents a serious challenge to the self–nonself theory which has sometimes been called the immunological paradox of pregnancy. In this paper I shall defend the self–nonself theory against the critique posed by Thomas Pradeu. In addition I critically examine the alternative he proposes—the continuity theory. The main problem I will raise for any theory of immunology is that the phenomenon to be explained in pregnancy is the non-rejection of the foetus despite the triggering of the gestator’s immune system. I will argue that contemporary modified versions of the self–nonself theory can rise to this challenge but that continuity theory cannot. As such, the immunology of pregnancy gives us a good reason to favour contemporary self–nonself theory over the continuity theory.