Side-chain theory

Ehrlich theorized from very early in his career that chemical structure could be used to explain why the immune response occurred in reaction to infection.

He hypothesised that antibodies bind to antigens through special chemical structures that he called "side chains" (which he later named "receptors").

Borrowing a concept used by Emil Fischer in 1894 to explain the interaction between an enzyme and its substrate, Ehrlich proposed that binding of the receptor to an infectious agent was like the fit between a lock and key.

According to this theory, the surface of white blood cells is covered with many side chains that form chemical links with the antigens.

According to Ehrlich, an antibody could be considered an irregularly shaped, microscopic, three-dimensional label that would bind to a specific antigen but not to the other cells of the organism.