Michael Heidelberger (April 29, 1888 - June 25, 1991) was an American immunologist, one of the fathers of modern immunology and the founder of immunochemistry. By applying the quantitative methods of analytical chemistry to the study of the mammalian immune system, he moved immunology from the imprecise realm of contemporary colloidal chemistry into the exact field of biochemistry, thereby laying a path for a new understanding of infectious diseases, their treatment, and their prevention. He devoted himself to exploring the implications of his path-breaking discovery, made with Oswald T. Avery in 1923, that type-specific antigens of pneumococcus bacteria, the most common cause of pneumonia, are complex polysaccharides. He showed that antibodies are proteins and that antibodies and antigens are multivalent, meaning that they have two or more binding sites. He used these findings to develop a simple vaccine against pneumonia whose effectiveness was first proven in WWII soldiers, and which in modified form remains in use today. He published over 350 scientific articles, lived to be 103, and was in the laboratory until a few weeks before his death in 1991.