Models for biological research: The theory and practice of Paul Ehrlich

In the late-nineteenth and early-twentieth centuries, the study of the living world placed considerable reliance on applied chemistry and chemical concepts. Paul Ehrlich, the subject of this paper, adopted from the mid-1870s principles of organic chemistry to devise procedures for use in histology, to suggest features of cellular structure, and to draw up cartoons that explained the nature of immunity. From a distinctly molecular basis, he devised meaningful experimental strategies and was inspired to develop speculative but effective theoretical models along the same lines that chemists had used to resolve problems of molecular structure and behavior. Thus, Ehrlich suggested that combining power and toxicity were independent properties of the antigen toxin. He succeeded to classify synthetic dyestuffs as stains, to use them for investigating molecular combustion, to adapt theories of dyeing to develop models for both combustion and immunity, and to exploit the properties of dyes in the development of chemotherapy. Ehrlich not only speculated on the behavior of synthetic chemicals toward his model of the protoplasm but also invoked biological specificity, a concept that would have a tremendous impact on immunology, drug development, and molecular biology.