1. With naturally or artificially attenuated infectious. agents of similar kind; or

2. With devitalized cultures of the micro-organisms concerned.

So far we know of two entirely different causes for this immunity.

A.-Poison Resistance (Specific Poison Immunity).

After recovery from a series of infectious diseases (diphtheria, tetanus, botulism) which have this in common, that during their course very active poisons are produced by the bacteria, there are found in the blood, and especially in the serum, characteristic substances which are actively antagonistic to the poisons (antitoxins). The antitoxic serum protects as well if it is used before the introduction of the poisonous culture (it immunizes passively) as if it is injected at the time of, or subsequent to, the infection (it heals). It operates as well against the introduction of the toxins of the concerned bacteria as (in larger doses) against the introduction of living cul

tures.

As yet little is known regarding antitoxins, but they are more resistant than the alexins to injurious influences. Thus the tetanus antitoxin bears very well without being destroyed: a temperature of 60°, also 70° to 80° for a shorter time, the action of sunlight (yellow better than blue rays), and putrefaction. Brieger and Ehrlich have obtained diphtheria antitoxin from the milk of goats, immune to diphtheria, in solid form,—whether it is an albuminous body or adheres to albuminous bodies, is still unknown. The antitoxins are best separated by means of zinc chlorid, but so far can not be freed from the final traces of zinc (Brieger and Boer, Z. H. XXI, 266).

Regarding the action of antitoxins and toxins upon each other the following is now known. As Behring and Kitasato supposed, a toxin solution in a test-tube with a sufficient quantity of antitoxin added is completely inactive, because toxin and antitoxin mutually neutralize each other (somewhat as base and acid). Buchner explained

this neutralization as only apparent, and maintained that both antitoxin and toxin continue together, but mutually hide each other because they influence the same animal organism in opposite ways, somewhat like atropin and muscarin. Now Buchner (Münch. med. Wochenschr., 1899, 523), especially since the work of Knorr (Z. H. xiii, 407), has accepted essentially the views of Behring. The principal proofs for these views are: (1) Dilute solutions of toxin and antitoxin neutralize each other more slowly than concentrated ones (in higher dilutions many chemical reactions occur slowly). (2) Equalized toxin and antitoxin mixtures after heating or long storage remain non-toxic, while toxin is much more resistant than antitoxin. (3) Animals into which are injected subcutaneously corresponding mixtures of toxin and antitoxin, after two hours show no antitoxin in the blood, although the toxin is much more rapidly absorbed. Finally, it is very noticeable that ricin (poisonous albuminous body from ricinus seeds) and antiricin act also antagonistically upon dead bodies in vitro (diluted blood) as if they neutralized each other; the antiricin increases the coagulating action of ricin (Ehrlich, Fort. der Med., 1897, 41). Similarly rattlesnake poison (cobra toxin) in vitro dissolves red corpuscles, while cobra antitoxin hinders this action. Stephens-Meyers (London Lancet, Mar. 5, 1898, 644).

As to opposing statements: Take an animal that is protected against a mixture of toxin and antitoxin, but not also protected against two, four, six, or ten times the quantity of the mixture (Bomstein and others). Cobbett and Santhack assert as their positive opinion, at least in the se of diphtheria, that, if the toxin has been completely utralized, even ten times the quantity of the mixture ould be tolerated (C. B. XXIV, 129), but, naturally, if he mixture contains a slight excess of toxin, ten times the quantity would be injurious.

Reference must be made to the incompletely understood fact that to a neutralizing mixture of toxin and antitoxin a very large quantity of toxin must be added before slight toxic action results from its injection, but that also very large quantities of antitoxin are required to again overcome this weak toxic action.

For the explanation of the origin and action of antitoxin in the human body, Ehrlich first expressed the idea that the antitoxins are exactly identical with those constituent parts of the poison-susceptible cells which are injured by the poison. The antitoxins are the "toxophoric side chains" or, more simply, according to Blumenthal, the toxin-binding group" of the toxin-susceptible albumin molecule.

46

The recent experiences of Wassermann, Behring, Blumenthal, Metschnikoff and Maria, and especially of Knorr (Münch. med. Wochenschr., 1898, Nos. 11 and 12), accord very well with this explanation. Thus, what occurs in the case of tetanus may be presented somewhat as follows:

If one introduces tetanus poison into an animal susceptible to tetanus (guinea-pig), after a time the poison disappears from the blood and becomes insolubly fixed by the chemical constituents of the ganglia of the spinal cord, and is therefore no more obtainable from the spinal cord. The binding of the poison by the spinal cord leaves it diseased; the poison-binding cells become functionally incapacitated. Wassermann could directly demonstrate that the spinal cord (and brain) can bind tetanus poison, since he showed that mixtures of tetanus toxin and emulsion of spinal cord are non-toxic. It is further interesting that the cord of animals susceptible to tetanus alone possesses this property, the cord of hens, which are immune to tetanus, not at all. The side chains are absent here, and the hen is insusceptible to tetanus for the same reason that its cord is without effect upon the toxins. On the contrary, the cords of animals dying from tetanus contain sufficient antitoxin to protect other animals from the disease. This is no objection, if the first animal died when a certain part of its spinal cells were poisoned through the union with toxin and long before all the poison-binding affinities of its spinal cord were satisfied.

Knorr has shown the identity of the antitoxin and the poison-fixing substance of the spinal cord by demonstrating that both possess the same susceptibility to injurious influences.

Where, by careful repeated poisoning of an animal until the surplus supply of antitoxin is dissolved in the serum,

the phenomena may be represented, according to Ehrlich, somewhat as follows: If, with the first cautious introduction of a small amount of tetanus poison only a small part of the toxophoric side chains is fixed, then only a few cells are injured, or many cells are so slightly injured that no disease symptoms occur. The side chains united with toxin are thrown off and replaced by the cells. This repair is always accomplished more rapidly, the more frequently and extensively the side chains are torn off by the toxins. Finally, the cells produce more side chains than can remain connected with them, and they are discharged into the blood as antitoxin. Knorr finds this explanation of the facts unsatisfactory, and claims that, while in one part of the diseased organism the antitoxin groups are fixed by toxin, the unpoisoned cells containing antitoxin are stimulated to a larger production of this substance.

From this essential independence of antitoxin from the chemical composition of the toxin one can understand that a specificity of antitoxin does not strictly exist. For example, tetanus and rabies antitoxin also protect against cobra poison (Calmette, A. P. IX, p. 225), and, according to Tizzoni, sterile non-toxic cultures of pneumococcus in rabbits' blood protect against tetanus poison (C. B. xxiv, 904).

The value of antitoxic sera is expressed as follows:

1. After Behring. He designates as a normal poison a oxin solution of which 0.01 c.c. is sufficient to kill a inea-pig weighing 250 gm. within four days. The toxin always injected in 4 c. c. of water just beneath the skin. Of this normal diphtheria toxin (DTN) 1 c.c. is sufficient. to kill one hundred guinea-pigs, each weighing 250 gm., or 25,000 gm. in weight of guinea-pigs. Behring expresses this as follows:

The DTN has a working value of 25,000; it kills 25,000 times its weight of guinea-pigs. A toxin ten times as strong is represented as DTN10; a ten times weaker one, as

DTN

10.

The quantity of antitoxin that is required to just protect 25,000 gm. weight of guinea-pigs from the minimal fatal dose of poison is called one immunizing unit.

If an immune serum contains in 1 c.c. one immunizing unit (IE), -i. e., neutralizes 1 c. c. DTN,-then it represents a normal antitoxin (DAN).

To determine the strength of an immune serum, 1 c. c. of normal toxin is mixed with increasing quantities of the serum; the quantity of the serum which suffices to neutralize it-for example, 0.1 c.c.-contains one immunizing unit, or the serum contains 10 IE to the c. c., and is then ten times DAN, and is represented thus, DAN10.

To cure a sick man, usually 600 to 1800 IE are employed, which are contained in 2, 4, or 6 c. c.; then it is a DAN300 that is used.

Recently a dried DA has been produced, of which 1 gm. contains as much as 5000 IE; of this about 0.125 gm. suffices for a single healing dose.

2. After Ehrlich. Ehrlich has recently introduced in the institute for testing serum, as a standard for determining values, a very durable dry antitoxin, which contains 1700 IE in 1 gm. A test-toxin is prepared corresponding to this antitoxin, and with this toxin the strength of the unknown serum is titrated. For the crude estimation of the working value of a serum, a toxin can be prepared corresponding to a higher serum of guaranteed strength of IE, and the unknown serum be titrated with this toxin. Because of the numerous cautions to be observed, serviceable results can be obtained only by experts. Compare Ehrlich, Klin. Jahrbuch, Bd vi.

B. Bacteria Resistance (Specific Bacterial Immunity).

While the antitoxins antagonize the toxins of the bacteria in an active manner, they are able, according to older observations not at all, according to more recent observations1 only to a slight extent, to kill bacteria―i. e., to act as bactericides. On the contrary, in a second group of infectious diseases (typhoid, cholera, swine erysipelas) the immunity depends upon the bactericidal action of the

1According to van de Velde (C. B. XXII, 527), strong antidiphtheria serum also possesses considerable bactericidal action; similar double action is presented also by various other immune sera; for example, antipyocyaneus serum. Compare also Emmerich and Löw, p. 110.