In 1890, another German researcher Emil von Behring, discovered that the blood of animals infected with different diseases contained chemicals that attacked the disease cells.
He called these chemicals antibodies and found that they often provided immunity, or lifetime protection, against a disease after the first attack.
Paul Ehrlich realized that this was how his dyes worked, the dyes only colored certain cells, and the antibodies attacked only certain kinds of cells without harming other tissue. He started working with Behring to find how antibodies work and if they could somehow be produced outside the body.
Ehrlich discovered that foreign bodies, like bacteria or poisons, contain protein molecules he called antigens.
When the human body senses that an antigen is present, it makes antibodies to make the substance harmless. Ehrlich called the battle between the two substances an immune response and wondered if he could make an organism produce antibodies by deliberately infecting animals with certain germs.
Behring and Ehrlich infected some horses with the bacteria that cause diphtheria, a very contagious, often fatal, respiratory disease that usually attacks children. Blood was then taken from the horses, and the antibodies were concentrated into an antitoxin.
An antitoxin is a medicine that contains antibodies to neutralize the toxic, or poisonous, effect of bacteria. This antitoxin medicine could be injected into humans to create immunity to the disease without contracting the disease first.
Ehrlich later explained exactly how antibodies work in his side chain theory.
He said that antibodies grew many “arms,” which he called receptors, to grab the foreign bodies. The structures of the antigen in the foreign body and the antibody receptors fit together like a lock and key.
This is how the antibodies destroy bacteria without harming surrounding tissue. Sometimes the receptors break off from the antibodies and attack foreign bodies by themselves.
Ehrlich had found his “magic bullets.”
Ehrlich once caught tuberculosis while studying the bacterium that causes the disease. He had to spend two years in the warm, dry air of Egypt to recover.