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).