The general characteristics of toxins have been described (Chapter XII). It has been stated that they are more or less specific in their action on cells. In order to affect a cell it is evident that a toxin must enter into chemical combination with it. This implies that the toxin molecule possesses a chemical group which can combine with a receptor of the cell. This group is called the _haptophore_ or combining group. The toxic or injurious portion of the toxin molecule is likewise spoken of as the _toxophore_ group. When a toxin is introduced into the body its _haptophore_ group combines with suitable _receptors_ in different cells of the body. If not too much of the toxin is given, instead of injuring, it acts as a chemical stimulus to the cell in the manner already described. The cell in response produces more of the specific thing, which in this instance is more receptors which can combine with the toxin, _i.e._, with its haptophore group. If the stimulus is kept up, more and more of these receptors are produced until an excess for the cell accumulates, which excess is excreted from the individual cell and becomes free in the blood. These free receptors have, of course, the capacity to combine with toxin through its haptophore group. When the toxin is combined with these free receptors, it cannot combine with any other receptors, _e.g._, those in another cell and hence cannot injure another cell. These free receptors constitute, in this case, _antitoxin_, so-called because they can combine with toxin and hence neutralize it. Antitoxins are specific--that is, an antitoxin which will combine with the toxin of _Clostridium tetani_ will not combine with that of _Corynebacterium diphtheriæ_ or of _Clostridium botulinum_, or of any other toxin, vegetable or animal. When a toxin is kept in solution for some time or when it is heated above a certain temperature (different for each toxin) it loses its poisonous character. It may be shown, however, that it is still capable of uniting with antitoxin, and preventing the latter from uniting with a fresh toxin. This confirms the hypothesis that a toxin molecule has at least two groups: a combining or _haptophore_, and a poisoning or _toxophore_ group. A toxin which has lost its poisonous property, its toxophore group, is spoken of as a _toxoid_. The theory of antitoxin formation is further supported by the fact that the proper introduction of _toxoid_, the _haptophore_ group, and hence the real stimulus, can cause the production of _antitoxin_ to a certain extent at least.