Nearly 65 years have passed since Peter Medawar posed the following question: “How does the pregnant mother contrive to nourish within itself, for many weeks or months, a fetus that is an antigenically foreign body.” Now, understanding of reproductive immunology has demonstrated that the HLA antigens in the placenta are non‐classical and do not induce rejection. In the placenta and in tumors, 50% or more of the cells are cells of the immune system and were once thought to be primed and ready for killing tumors or the “fetal transplant” but these cells are not potential killers but abet the growth of either the tumor or the placenta. We believe that these cells are there to create an environment, which enhances either placental or tumor growth. By examining the similarities of the placenta's and tumor's immune cells, novel mechanisms to cause tumors to be eliminated can be devised.