Letter from Michael Heidelberger to Joshua Lederberg

In this exchange between Heidelberger and the molecular geneticist Joshua Lederberg, the two agree on the need to elucidate in detail the chemical composition and three-dimensional structure of pneumococcal and other antigenic polysaccharides, a need Heidelberger was addressing at the time through his examination of immunological cross-reactions. Such cross-reactions, in which antigens react with certain non-homologous antibodies (antibodies formed against another antigen with which the first antigen shares closely-related or identical antigenic determinants, a site on the surface of an antigen molecule to which a single antibody binds), told organic chemists like Heidelberger much about the arrangement of the sugars in the repeating chemical units of polysaccharides, which in turn determined their immunological properties.