Letter from Michael Heidelberger to Enrique E. Ecker, Western Reserve University

In this letter, Heidelberger proposed a uniform terminology for the young field of complement research, a field Heidelberger helped found during the late 1930s and early 1940s by proving that complement was made up of a group of specific chemical substances that could be isolated in the laboratory. Subsequent research has shown that complement is a complex system of over twenty serum proteins that after activation play an essential enzymatic role in host defense mechanisms against invading microorganisms, namely in the promotion of inflammation, phagocytosis (the engulfing of invading organisms by immune cells), and lysis (the breaking up of such organisms).