After a short introduction concerning immune system function, the physiological and pathophysiological aspects of the foeto maternal relationship are discussed. Ontogeny of the immune system, the foetus as an in utero 'foreign body', and maternal immune competence during pregnancy and lactation are all considered. The process of presentation of maternal antibodies to the foetus/neonate is then presented. The phenomenon of pinocytosis is also discussed. The article concludes with a discussion of the problem of neonatal infections.