So far it has been shown that the best defence of the body is, as is the best defence in war, by offensive measures, as illustrated by phagocytosis and destruction by the serum.  Both of these actions can be increased by their exercise just as the strength of muscular contraction can be increased by exercise, and the facility for doing everything increased by habit.  Certain of the infectious diseases are, as has been said, essentially toxic in their nature, and in cultures the organisms produce poisonous substances.  By the injection into the tissues of such substances the same disturbances are produced as when the bacteria are injected.  Such a disease is diphtheria.  In this there is only a superficial invasion of the tissues.  The diphtheria bacilli are located on the surface of the tonsils or pharynx or windpipe, where, as a result of their action, the membrane so characteristic of the disease is produced.  The membrane may be the cause of death when it is so extensively formed as to occlude the air passages, but the prominent symptoms of the disease, the fever, the weakness of the heart and the great prostration are due not to the presence of the membrane, but to the action of toxic substances which are formed by the bacteria growing in the superficial lesions and absorbed.  Tetanus, or lockjaw, is another example of these essentially toxic diseases.  The body must find some means of counteracting or destroying these injurious toxic substances.  It does this by forming antagonistic substances called antitoxines, which act not by destroying the toxines, but by uniting with them, the compound substance being harmless.  It has been found that the production of antitoxine can be so stimulated by the injection of toxine that the blood of the animal used for the purpose contains large amounts of antitoxine.  The horse is used in this way to manufacture antitoxine, and the serum injected into a patient with diphtheria has a curative action, a greater amount being thus introduced than the patient can manufacture.

[Illustration:  FIG. 18.—­DIAGRAM TO ILLUSTRATE EHRLICH’S THEORY OF ANTITOXINE FORMATION.  The surface of the cell (n) is covered with receptors some of which (b) fit the toxine molecule, (a) allowing the toxine to act upon the cell.  Under the stimulus of this the cell produces these receptors in excess which enter into the blood and there combine with the toxine as in a^1 b^1, thus anchoring it and preventing it from acting upon the cells.  The receptors c and d do not fit the toxine molecule.]

A very ingenious theory which well accords with the facts has been given by Ehrlich in explanation of the production of antitoxine and of the reaction between toxine and antitoxine (Fig. 18).  This is based on the hypothesis, which is in accord with all facts and generally accepted, that the molecules which enter into the structure of any chemical substance have in each particular substance a definite arrangement,