Immunology is a fast developing and intriguing biomedical science, which can give rise to specific considerations about the physiological process of both successful and unsuccessful vivparous pregnancy. It is normal in clinical organ transplantation for unmatched foreign tissues (allografts) to provoke immunological rejection by the host, unless there has been prior tissue matching (histocompatibility antigen tissue typing) or immunosuppressive therapy. Thus, it is still not fully clear how, after ‘random’ mating, haplo-nonidentical fetal tissue is able to survive in the potentially hostile immunocompetent maternal environment. The majority of pregnancies survive uninterrupted and there has now been much speculation and research regarding the immunological success of pregnancy (i.e. nature’s transplant). Medawar orginally offered four nonexclusive hypotheses to explain the enigmatic immunological survival of normal pregnancy:

1) the conceptus is not immunogenic and therefore does not evoke an immunological response;

2) pregnancy alters the maternal immune response;

3) the uterus is an immunologically privileged site;

4) the placenta is an immunological barrier between the mother and the as yet immunologically incompetent fetus.

Before discussing these, as well as some of the clinical immunological problems that may arise during pregnancy, it is necessary to outline some of the basic components of the normal immune system. This will lead to a description of current understanding of immunological events at the fetomaternal interface as well as the maternal immune response in human pregnancy.

Pregnancy results in profound physiological changes in the cardiovascular system, yet these changes are completely reversible. It is apparent that vaso-active factors, some as yet probably unidentified, which act as humoral or local autocrine or paracrine regulators of vasular resistance, play a major role in these cardio-vascular changes. This role may be heightened in pregnancy when there has to be a large increase in blood flow to the uterus and placenta while maintaining adequate flow to other vascular beds. Our knowledge of the mechanisms of action of these vaso-active factors and their interactions with each other still remains incomplete. Alterations in synthesis and action of these vaso-active factors may occur in pregnancies associated with pregnancy-induced hypertension, pre-eclampsia or intra-uterine growth retardation. Investigation of such alterations may help to elucidate the roles of vaso-active factors in both normal and pathological situations. The gestational hormones oestrogen and progesterone, are obviously prime candidates as overall regulators of the cardiovascular changes of pregnancy and as agents which alter the synthesis or action of other vaso-active factors. Currently, much attention is being focused on the role of local autocrine or paracrine vaso-active factors which may be produced by the endothelium or by the underlying vascular smooth muscle cells and alterations in their production or action in the hyptertensive disorders of pregnancy. The endothelium forms the largest endocrine organ within the body and so its importance in the mediation of vascular events should not be under-estimated. The principal objective of this review is to examine the roles of these many autocrine and paracrine vaso-active factors during pregnancy and their relation with the overall regulation of the vascular system. Changes which may occur and be involved in the aetiology of pre-eclampsia and growth retardation will also be examined.