The immune system is primed to respond to disease-bearing microbes of almost any description, but what was far from clear was exactly what measures it uses to defeat any such attack. The approaches that the two recipients of the 1908 Nobel Prize in Physiology or Medicine took to uncovering the answer differed in philosophies and in route of investigation, but in the process Ilya Mechnikov and Paul Ehrlich revealed how our internal defence system is armed with more than one protection tactic.

While studying the manner in which cells in simple marine organisms absorb nutrients Ilya Mechnikov developed his theory for the destruction of unwanted intruders. He discovered that inserting parts of a rose thorn into starfish larvae provoked a particular kind of cell to engulf and dispose of the offending object. After seeing these wandering cells in other organisms, such as water fleas, causing similar destructive responses, Mechnikov made the bold conceptual leap of proposing that this reaction to extraneous matter represented a fundamental defence mechanism. In a process that he termed phagocytosis, special cells called phagocytes are deployed by the host to catch and destroy harmful bacteria or to inhibit their further development.

Paul Ehrlich thought that the immune system’s weaponry could be found in blood, which led him to formulate a theory explaining how antibodies seek and neutralize the toxic actions of bacteria. Drawing inspiration from chemistry, in particular a popular hypothesis stating that an enzyme and the substance upon which it acts fit together precisely like a lock and key, Ehrlich imagined the cell to be surrounded by protruding side-chains, through which it could collect and draw in nutrients and other valuable material. His theory proposed that cells produced scores of extra side chains to counter the threat of any perceived bacteria attack. Breaking off and circulating in the blood stream, these antibody side chains bound in a highly specific manner to poisons created by bacteria to prevent them from reaching their intended targets, thus rendering the host immune from their ill-effects.