The Croonian lecture. -- On immunity with special reference to cell life.

Ehrlich's "lock and key" theory of antibody antigen relationship, or interaction, in which the toxin molecule (i.e the antigen) binds to the cell receptor, and then the antibody binds in a lock and key manner to the toxin (antigen) to neutralize the toxin. 

Digital facsimile from royalsocietypublishing.org at this link.

(Thanks to Juan Weiss for this reference and its interpretation.)