Abstract
CONFRONTATION of mammals with tissues or cells of alien genetic constitution is not restricted to the professional activities of surgeons and experimental biologists. Every pregnancy in an outbred population represents a natural parabiotic union and intimate mutual exposure of mother and fetus to each other's tissues and foreign transplantation antigens. This follows since the fetus inherits from its father a variable number of transplantation antigens that are foreign to the mother, and fails to inherit from its mother all her histocompatibility factors.The purpose of this article is to consider the relation between mother and fetus from the point of . . .