The "opsonin," or "relish," is something exuded into or produced in the blood fluid when the attacking microbe arrives.
opsonin is what you butter the disease germs with to make your white blood corpuscles eat them.
In the case of diphtheria, the antitoxin appears to be more efficacious than an opsonin.
opsonin op·so·nin (ŏp'sə-nĭn)
An antibody in blood serum that causes bacteria or other foreign cells to become more susceptible to the action of phagocytes.