and that in a compound substance each elementary substance entering into the compound molecule has chemical affinities, most of which may be satisfied by finding a suitable mate.  Ehrlich assumes that the very complex chemical substances which form the living cells have many unsatisfied chemical affinities, and that it is due to this that molecules of substances adapted for food can enter the cells and unite with them; but there must be some coincidence of molecular structure to enable the union to take place, the comparison being made of the fitting of a key into a lock.  The toxines—­that produced by the diphtheria bacillus being the best example—­are substances whose molecular structure enables them to combine with the cells of the body, the combination being effected through certain chemical affinities belonging to the cells termed receptors.  Unless the living cells have receptors which will enable the combination with the toxine to take place, no effect can be produced by the toxine and the cells are not injured.  This is the case in an animal naturally immune to the action of the diphtheria bacillus or its toxines.  In the case of the susceptible animal the receptors of the cells of the different organs combine with the toxine to a greater or less extent, which explains the fact that different degrees of injury are produced in the different tissues; the toxine of tetanus, or lockjaw, for example, combines by preference with the nervous tissue, that of diphtheria with the lymphatic tissue.  It is known that in accordance with the general law of injury and repair, a loss in any part of the body stimulates the tissue of the same kind to new growth and the loss is thus repaired; it is assumed that the cell receptors which combine with the toxine are lost for the cell which then produces them in excess.  The receptors so produced pass into the blood, where they combine with the toxine which has been absorbed; the combination is a stable one, and the toxine is thus prevented from combining with the tissue cells.  The antitoxine which is formed during the disease, and the production of which in the horse can be enormously stimulated by the injection of toxine, represents merely the excess of cell receptors, and when the serum of the horse containing them is injected in a case of diphtheria the same combination takes place as in the case of receptors provided by the patient.  In the case of the destruction of bacteria in the blood by the action of amboceptor and complement, the amboceptor must be able to combine with both the bacterial cell and the complement which brings about its destruction, and just as antitoxine is formed so new amboceptors may be formed.

Few hypotheses have been advanced in science which are more ingenious, in better accord with the facts, have had greater importance in enabling the student to grasp the intricacies of an obscure problem, and which have had an equal influence in stimulating research.  The immunity which results from disease in accordance with this theory, is due not to conditions preventing the entrance of organisms into the body, but to greater aptitude on the part of the cells to produce these protective substances having once learned to do so.  An individual need not practise for many years, having once learned them, those combinations of muscular action used in swimming; but the habit at once returns when he falls into the water.