Facilitation reaction (enhancing antibodies and suppressor cells) and rejection reaction (sensitized cells) from the mother to the paternal antigens of the conceptus.

Humoral and cellular immune agents of a maternal reaction were investigated during pregnancy. Fluorescence studies performed on mouse placentae at 14 days detected maternal immunoglobulins of mainly IgG1 but also IgG2 subclasses. These immunoglobulins, after acid elution, can rebind the placenta and the thymocytes of the relevant paternal strain in case of allogeneic pregnancies, demonstrating an antibody activity towards both placenta specific and paternal strain antigens. They can specifically enhance a paternal strain tumour allograft on a maternal strain recipient. Spleen cells from an allogeneically pregnant mother can reduce or promote paternal strain tumour allograft on a maternal strain recipient. The aggressive effect is shown with small doses of transferred cells, whereas large doses promote enhancement. The suppression of the cytotoxic response of the recipient was ascribed to T cells by use of anti-theta plus complement. Thus, both the rejection reaction (immune cytotoxic cells) and the facilitation reaction (enhancing antibodies and suppressor cells) were demonstrated during pregnancy.