SERUM THERAPY. Serum therapy consists in the intro duction into the patient of blood serum containing specific anti bodies, preformed in some other animal, which will (a) neutralise the toxins produced by the corresponding infecting bacteria or (b) destroy the corresponding bacteria themselves—thus estab lishing a condition of "passive" immunity (q.v.).
Again, speaking generally, natural recovery from a bacterial in fection due to an organism of the first group coincides with the appearance in the blood serum of a neutralising agent known as anti-toxin in adequate amount ; whilst when dealing with microbes of the second class, nature elaborates, also in the serum, a different anti-body—a bacteriolysin or immune body for the purpose of de stroying the invading bacteria. In each instance the resulting anti-body is specific in character—that is to say, it will only act in opposition to the particular toxin in the one case or in the other to the members of that particular species of microbe whose entry into the body provoked its formation—the serum which is the vehicle of the curative substance is termed in this first instance "anti-toxic" and in the second "anti-bacterial." Thus the early and accurate recognition of the bacterium responsible for any infec tion it is proposed to treat determines the selection of the appro priate anti-serum.
concentrated, some of its unneeded proteins removed and purified.
It is obvious that a correct appreciation of its valency is neces sary for the scientific employment of, any serum in the treatment of disease ; but, unfortunately, bacterial poisons formed by various micro-organisms, and even by different strains of the same species, not only exhibit many variations in toxicity depending upon bi ological factors (not all of which are at present known) but they have, so far, proved incapable of isolation in a state of chemical purity. Consequently, it is only possible in the majority of in stances to effect a comparison by observing the results of their introduction into suitable experimental animals.
Indeed only in the case of diphtheria toxin is there any uni versally accepted standard, for here Ehrlich prepared, many years ago, a standard diphtheria anti-toxin of constant valency which is preserved hermetically sealed in vacuo, by means of which all diphtheria toxins can be graded and the potency of the sera pre pared by their means expressed in anti-toxic units.
In the case of toxins produced by other bacteria, and in the preparation of anti-bacterial sera, the plan usually adopted is to inoculate gradually decreasing doses of the virus into susceptible animals until the smallest quantity that will cause death with cer tainty, and within a specified time, is determined. This is termed the minimal lethal dose (M.L.D.) and forms the basis for the standardisation of the corresponding anti-serum. Varying quanti ties of serum are in turn mixed with a constant dose of virus— usually ioo M.L.D.—and injected into animals until that amount of serum is determined which will effectively neutralise the test dose. This quantity of serum is then regarded as containing one unit of anti-body and the serum which is in process of standardisa tion is stated to contain so many units per cubic centimetre.