The opsonic index is a historical immunological measure expressing the power of a person’s blood serum to prepare bacteria for destruction by phagocytes, relative to a normal standard. Serum is mixed with bacteria and white blood cells, and the number of bacteria engulfed is compared against control serum; the ratio is the index. Developed in the early twentieth century by Almroth Wright, it was used to assess immunity and to guide vaccine therapy, with a low index suggesting susceptibility. The concept rests on opsonins, serum proteins including antibodies and complement components that coat pathogens and mark them for ingestion. Though superseded by modern assays of antibody titres and cell-mediated immunity, the opsonic index remains a landmark in immunology’s history and in understanding how nutrition, through protein-energy status and micronutrients, supports immune defence.