<prompt> Opsonic index is related to -PhagocytosisVasodilatationApoptosisNecrosis  <response>: Ans. is a i.e., Phagocytosis The coating of an antigen or particle eg - infectious agent by substances, such as antibodies, complement components and fibronectin facilitate phagocytosis. This is called opsonization. The substance which enhances phagocytosis causing opsonization is called opsonin. Two most important opsonins are antibodies IgM, IgG and complement C3b. IgM is more effective than IgG in causing opsonization. Other opsonins are serum proteins like fibrinogen, mannose binding lectin and C-reactive proteins. The term Opsonic index is defined as the ratio of of the phagocytic activity of patients blood for a particular bacterium to the phagocytic activity of blood of normal individual. It is used to study the progress of resistance during the course of the disease. 