In 1882, the Russian scientist Ilya Mechnikov was working in Messina, Italy, studying the larvae of the sea star. When he inserted a thorn into a larva, something weird happened. Mechnikov noticed strange cells gathering at the point of insertion. The cells surrounded the thorn, eating any foreign substances that entered through the ruptured skin. Mechnikov was thrilled. He decided to name these new cells phagocytes from the Greek words meaning "devouring cells."

The discovery of the phagocyte cells was very important since it helped scientists understand the concept of immunity and how the body defends itself against disease. If the phagocyte cells come upon anything alien, they absorb it and destroy it. The phagocytes also play an important role in activating the rest of the immune system.

At the end of the nineteenth century, the German scientist Paul Ehrlich developed the "side-chain theory" to explain immunity and how antibodies were formed. Although we now know that some of his ideas were incorrect, this theory allowed him to accomplish important work and provided the groundwork for later researchers in this field.

Ehrlich argued that all cells have a wide variety of special receptors that he called side-chains. He thought that these receptors worked like gatekeepers or locks for the cell. Each receptor/side-chain had a unique structure, and only substances matching this structure were allowed to enter the cell.

The side-chain receptors’ primary function was to absorb nutrients for the cell. Unfortunately, the receptors also allowed many toxic substances to enter. According to Ehrlich, the body defended itself against these toxins in the following way: When a cell was attacked by a toxin, it started to produce excess side-chains matching the toxin. These excess side-chains then were released, flooding the body and neutralizing free toxins by attaching to them. The toxin was wiped out and remaining healthy cells protected.

Pioneers of Immune System Research

Ehrlich had hit upon the key concept that the body produces substances, which we today call antibodies, to help in the destruction of invaders, while Mechnikov had discovered that certain body cells could destroy pathogens by simply engulfing or "eating" them. We now know that the incredibly complex immune system mounts attacks in both of these ways. It's entirely fitting, therefore, that Ilya Mechnikov and Paul Ehrlich shared the 1908 Nobel Prize in Physiology or Medicine in recognition of their work on immunity.

First published 8 November 2004