The immunizing effect of pregnancy was assessed using the cardiac allograft technique to estimate maternal reactivity against paternal antigens. In this study the following immunological activities were observed in pregnant or post partum mice. Cardiac allografts expressing paternal antigens were accepted in pregnant or post partum mice longer than in virgin recipients. The cardiac allograft
... [Show full abstract] enhancing effect was strain specific, was directly transferable by maternal lymphoid cells but not serum and could be shown to remain in effect in post partum female mice for at least five weeks after delivery.