5. _The Antitoxin Theory._ We have gathered, then, that whenever bacteria, introduced into the blood and tissues, fail to multiply or produce infection (as in saprophytic bacteria, or in immunity of a particular animal from a specific microbe), this inability to perform their rôle is brought about by some property in the living and normal blood serum which opposes their life and action; and further we have learned that this protective property is exhaustible according to the number of bacteria, and differs with various species of bacteria, and in different animals. Buchner designates these protective bodies, held in solution in the blood, _alexines_, and regards them as belonging to the albuminous bodies of the lymph and plasma. Where the blood and tissues do not possess this power, the animal is susceptible. Now, as we have already seen from the experiments of Ogata, Kitasato, and others, the blood of an animal dead of anthrax is protective against anthrax, from which and the foregoing it appears that microbes produce by their growth in the tissues poisonous substances we term _toxins_, which have the power of producing in the blood and body cells substances inimical to themselves, named _antitoxins_, and so long as these latter substances remain in the tissues the body remains insusceptible to further attacks of the same disease. Alexines are _naturally_ produced antitoxins; antitoxins are _acquired_ alexines. Hence we have the well-known terms "natural" and "acquired immunity." Of the former we have already spoken. _Acquired immunity_ is a protection not belonging to the tissues of individuals naturally and as part of their constitution, but it is acquired during their lives as a further accomplishment, so to speak, of their tissues. This may happen in one or both of two ways. Either it may be an involuntary acquired immunity, or a voluntary acquired immunity. For example, the former is at once illustrated by an attack of the disease. Small-pox, typhoid fever, even scarlet fever, are diseases which very rarely attack the same individual twice. That is because each of these diseases leaves behind it, on its first appearance, its antitoxic influence. Hence the individual has _involuntarily_ acquired immunity against these diseases. An example of voluntary acquired immunity is also at hand in the old method of preventive inoculation for small-pox, or _variolation_. This was clearly an inoculation setting up an artificial and mild attack of small-pox, by which the antitoxins of that disease were produced, and protected the individual against further infection of small-pox; that is to say, it was a voluntary acquired immunity. This form of artificial production of protection is generally called _artificial immunity_. Let us now marshal together these various terms in a table as follows: _Immunity_ in man {= a condition of protection of insusceptibility to { certain diseases.