The word "immune" derives from the Romans, who used it to describe those persons declared exempt from taxation and other obligations to the state. The meaning was later applied to the exemption and protection due to the early Christian church and its officers, as in "papal immunity" or the "immunity from arrest" found in the sanctuary of the church. The understanding of the word changed from the concept of "exemption" to the connotation "rendered safe," and usage implied that such protection was innate. "Immune" was thus the word applied to that state in which animals were protected or rendered safe from an infectious agent, either by prior infection or by experience. The substances which appeared in the serum under these circumstances were logically called "immune bodies"; they later became "antibodies," and again, logically, immunity —the state of refractoriness to infection— was equated directly by many persons with such circulating antibodies.*