Opsonization is a process by which a particulate antigen becomes more susceptible to phagocytosis when it combines with opsonin. The opsonin is a heat-labile substance present in fresh normal sera. Unlike opsonin, bacteriotropin is heat-stable substance present in the serum but with similar activities.
The term “opsonic index” is defined as the ratio of the phagocytic activity of patient’s blood for a particular bacterium to the phagocytic activity of blood from a normal individual. It is used to study the progress of resistance during the course of disease. It is measured by incubating fresh citrated blood with the suspension of bacteria at 37°C for 15 minutes and estimat-ing the average number of phagocytic bacteria from the stained blood films.