TITOXINE 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, 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 an