Blood Groups and Susceptibility to Disease

During subsequent years a large literature accumu lated, but it would probably be true to say that during nearly 30 years only one positive association emerged, and that not very clearly, for the numbers were relatively small and the result was obscured in the growing volume of negative and conflicting findings. It is instructive to look back in the light of our present knowledge and to consider what the difficulties were that delayed for 30 years the dis covery of associations that now seem so obvious and for which overwhelming evidence had rapidly been forthcoming. The principal difficulties may be listed as follows: