<prompt> Opsonic index is the ratio of ?C3b opsonin to antibody opsininPhagocytic activity of patient to phagocytic activity of normal individualSerum concentration of opsonin in patient to serum concentration in normal individualComplement activity in patient to complement activity in normal individual  <response>: Ans. is b i.e.,Phagocytic activity of patient to phagocytic activity of normal individual The coating of an antigen or paicle 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 impoant opsonins are antibodies IgM, IgG and complement C3b.1gM 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 paicular 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. 