For the last ten or fifteen years especially close attention has been given in medical research to ascertain how it is that the body itself resists disease and throws it off. Why is it that certain animals or groups of animals show great susceptibility to certain diseases, especially those of bacterial origin, and other groups show immunity to such infections? Why is it that one organism shows different degrees of resisting power at different times and under different conditions? What are immunity and susceptibility, and upon what factors do they depend? Over these essential questions have been working some of the greatest minds of modern medicine, and many truths have been discovered and principles established upon which is rapidly being built definite lines of practical therapy. The mortality of certain diseases has already been materially reduced because the underlying individual factors in their etiology have been found and these factors have been either destroyed entirely or their action restricted by definite methods based upon definite physiological, chemical, and pathological facts.

Among the many discoveries in such work which seem to be well-founded falls the subject of opsonins, and at the present timhere is no more "live" subject in the current literature than that of opsonins. That there are such chemical bodies in the blood-serum as opsonins has been pretty definitely settled. Just what bearing these bodies have upon the question of susceptibility and immunity is by no means yet settled. At the most they are only one factor among the many. Their comparative value or importance relative to the other factors is not determined. The discovery of opsonins and their evident importance; the attractive lines of research work that the study of opsonins offer, and the results of a practically new therapy against bacterial disease have attracted the attention of the medical world. To-day not only the laboratories are busy establishing the status of opsonins from a physico-chemical standpoint, but practitioners in practically all specialties are collecting clinical data from the vaccine therapy, that is based, as far as we yet know, upon opsonins.

As is well known, the investigator who has named these certain chemical bodies that are in the blood-serum opsonins is Sir A. E. Wright, of London. It is due to his observations that their identity was established and the practical technique of vaccine therapy based.

The subject of opsonins falls under the larger heading of phagocytosis when we consider the main divisions of immunity. The blood has three principal methods of resisting bacterial invasion: (1) A bacterial destroying action (bactericidal); (2) an antitoxic action, and (3) a phagocytic. In other words, the blood assists the body to oppose the pathogenic germs by killing some varieties; by neutralizing the poisons that other varieties secrete (such as the case of the diphtheria bacillus), and finally by destroying still other varieties by the devouring action of the white blood-cells.

Opsonins have only to do, as far as is known, with the last method, and in fact opsonins are chemical bodies in the serum that aid in the destruction of pathogenic germs by causing them to be devoured by the white blood-cells. The assistance is given in this way: The invading germ is first neutralized by these chemical bodies, opsonins, which come in contact with it, and only then can the germs be taken up by the white cells. Just what the interaction is between germ and opsonin is not known, but most investigators consider it best explained by the so-called Ehrlich's "side-chain theory." The active germ is supposed to possess numerous unsatisfied arms or radicles, by which it combines other cells, just as is the case in the simpler bodies of organic chemistry. Until these radicles are satisfied it has been found that the germ is not devoured by the phagocytic white blood-cells. The opsonins in the serum are chemical bodies that neutralize these arms, and when the germs are bathed with a serum containing opsonins, these germs become neutralized and can be and are then taken up and destroyed by the white blood-cells, evidently because all these attacking arms have been made powerless. To go into the theory a step further, it is thought that one method by which a pathogenic germ injures or destroys a living body-cell is first by "fastening" itself to the cell by these arms of union (the body-cell has like unsatisfied arms).

and then pouring into the cell-body its cell-destroying elements. The opsonins are supposed to come into this close chemical union with the germ. The germs are at least so neutralized or acted upon by the opsonins that they can now be taken up by the leucocyte when they could not have been taken up before they were so neutralized. In this way the opsonins aid phagocytosis, and their importance is very great if it be found that phagocytosis is the principal factor of immunity and susceptibility to bacterial insult. Many of the foremost investigators, like Metchnikoff, believe this is so. The word "opsonin" Wright coined because it means "to prepare food for "('oco;Eo). The opsonins prepare the food (germ) for the devouring cell (leucocyte).

Wright found that each variety of germ was neutralized by a definite variety of opsonins - i. e., the action of opsonins is specific.

Not only did Wright establish the identity of opsonins, but he also devised a method of estimating the amount or strength of them in any given serum. Further, he found that persons whose serum had, for example, a low opsonic content relative to one kind of germ, need not necessarily have a low content of opsonins which act upon some other kind of germ.

Again it was found that the serum from a person who had been suffering from some chronic infection was usually low in opsonic content as compared with the serum taken from a normal healthy individual; and likewise in acute cases of infection it was learned that the serum content of opsonins, relative to the specific bacterium causing this infection, did not remain from day to day within normal limits, but varied more or less. As the sera from many normal individuals were found to remain within definite boundaries, when examined at varying intervals, over long extent of time, this variation in the infected person's opsonic strength was considered significant.