The side-chain theory is a theory proposed by Paul Ehrlich (1854–1915) to explain the immune response in living cells. 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 believed that toxins and antitoxins were chemical substances at a time when very little was known about their nature. The theory explains the interaction of antibodies and antigens in the blood, and how antibodies are produced.

Paul Ehrlich was a Nobel Prize-winning German physician and scientist who worked in the fields of hematology, immunology, and antimicrobial chemotherapy. Among his foremost achievements were finding a cure for syphilis in 1909 and inventing the precursor technique to Gram staining bacteria. The methods he developed for staining tissue made it possible to distinguish between different types of blood cells, which led to the ability to diagnose numerous blood diseases.