The concept of reduced maternal immune responsiveness in pregnancy emerged when some studies demonstrated that the immune response to particular antigens is decreased throughout pregnancy. Manifold substances are stated to be immunomodulatory or immunosuppressive, such as the components of the seminal fluid as well as several regulatory and inhibitory molecules. Slight changes take place in the maternal immune competence in pregnancy, which makes sense since a generalized immunosuppression would probably result in the loss of fetus. Supposedly, enhancement of the immune system and the following immunomodulation in mother are the operative mechanism throughout pregnancy. Conceivably, the overall response of the immune system during pregnancy could be the net result of interactions which operate for ensuring the survival of the antigenically alien fetus and simultaneously prevent an excessive immunosuppression state. Therefore, such dynamic homeostatic mechanisms seem to be significant for a successful pregnancy.