Letter from Otto Bier, Instituto Biologico (Brazil) to Michael Heidelberger

Otto Bier recounted in this letter his return to Brazil from a year in Heidelberger's laboratory, where he had studied complement, a complex, essential component of host defense mechanisms against invading organisms. At the time, the term referred to the heat-sensitive factors in serum that trigger cytolysis, the dissolution of antibody-coated cells. Today, complement refers to a functionally related system of at least twenty different serum proteins that play a key role not just in cytolysis, but in other immune responses, including phagocytosis, the engulfing of foreign matter by immune cells, and anaphylaxis, a form of hypersensitivity reaction to a specific antigen with often life-threatening consequences. Heidelberger was one of the founders of the study of complement by showing that it was a group of chemical substances related to one another through enzymatic reactions.