their role in immune reactions. In 1927 he left the Rockefeller Institute to become head of the chemical laboratory at Mount Sinai Hospital. A year later he moved to the Columbia University College of Physicians and Surgeons. Columbia University His role as consulting chemist in its department of medicine suited his generous temperament. The door to his office, which he likened to "42nd Street and Broadway" because of its traffic, was open for anyone, especially junior researchers, to stop by, discuss matters of science or politics, and seek his advice. During his 27 years there he used his unique knowledge of polysaccharide antigen chemistry to develop methods, in particular the precipitin reaction, for isolating pure antibodies, which he proved were protein and which he measured in absolute units of weight for the first time. He and his collaborators Forrest E. Kendall and Elvin A. Kabat formulated a quantitative theory of precipitin and other immune reactions, which showed that such reactions unfolded in three distinct stages and which posited that antigens and antibodies were bi-or multivalent, meaning that they could combine in varying proportions. These findings enabled Heidelberger to develop a much more potent antiserum to meningitis in infants, as well as a simple but effective vaccine against several forms of pneumonia, which was successfully tested among Army Air Force recruits in 1944. Later life Upon his retirement from Columbia in 1954, Heidelberger moved to the Institute of Microbiology at Rutgers University, and in 1964 to the New York University School of Medicine. There he continued his research on pneumococcal polysaccharides and their cross-reactions with various types of antisera, always in pursuit of his lifelong objective to relate chemical structure to immunological specificity, working full-time until the age of 100, and then part-time until his death in 1991 at the age of 103. Heidelberger received fifteen honorary degrees and 46 medals, citations, and awards for his work, including two Albert Lasker Awards in 1953 and 1978, the Louisa Gross Horwitz Prize in 1977, the National Medal of Science from President Lyndon B. Johnson in 1967, and the Bronze Medal of the City of Paris in 1964. He was a member of the National Academy of Sciences and the New York Academy of Medicine, as well as an officer of the Légion d'honneur of France. He served twice as president of the American Association of Immunologists, in 1947 and 1949. Both times