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, 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.