Immunological and genetic factors influencing development and susceptibility to cancer.

The results of a variety of studies on the genetic and immunological aspects of reproduction can be integrated into a hypothesis about the factors that regulate implantation and development and that may also cause an increased susceptibility to cancer. The primary condition for successful reproduction is genetic compatibility between the mating partners: there must be no recessive lethal genes that could act alone or epistatically to cause embryonic or fetal death. Such recessive lethal genes have been identified in the mouse (t-haplotypes) and in the rat (grc), and there is some evidence that they also exist in humans. Immunological factors may modulate the implantation of the fertilized ovum under some circumstances after the genetic condition has been met. The same genetic factors that affect development may also affect susceptibility to cancer. This part of the hypothesis is supported by a number of clinical correlations between congenital defects and a higher incidence of cancer and by the demonstration of an increased susceptibility to the effects of chemical carcinogens in rats carrying the grc.