Final answer:
The side-chain theory of antibody formation is associated with Paul Ehrlich. His hypothesis contributed significantly to the early understanding of immune responses and the production of antibodies.
Step-by-step explanation:
The side-chain theory of antibody formation is associated with Paul Ehrlich. This theory, proposed by Ehrlich at the end of the 19th and beginning of the 20th centuries, suggested that cells could have side-chains that functioned as receptors for antigens. When an antigen bound to these receptors, the cell would then produce more of the specific side-chains to neutralize the antigen, leading to the formation of antibodies. Although Ehrlich's theory isn't entirely accurate by today's understanding, it played a significant role in advancing immunological research and understanding how the immune system might recognize and respond to pathogens.