The close relationship between toxins and enzymes has already been pointed out. This is still further illustrated by the fact that when enzymes are properly introduced into the tissues of an animal there is formed in the animal an _antienzyme_ specific for the enzyme in question which can prevent its action. The structure of enzymes, as composed of a _haptophore_, or uniting, and a _zymophore_ or _digesting_ (or other activity) group, is similar to that of toxins, and _enzymoids_ or enzymes which can combine with the substance acted on but not affect it further, have been demonstrated. These free cell receptors, antitoxins or antienzymes, which are produced in the body by the proper introduction of toxins or enzymes, respectively, have the function of _combining_ with these bodies _but no other action_. As was pointed out above, this is sufficient to neutralize the toxin or enzyme and prevent any injurious effect since they can unite with nothing else. Since these receptors are the simplest type which has been studied as yet, they are spoken of by Ehrlich as _receptors of the first order_. Other antibodies which are likewise free receptors of the first, order and have the function of combining only have been prepared and will be referred to in their proper connection. They are mainly of theoretical interest. Ehrlich did a large part of his work on toxins and antitoxins with _ricin_, the toxin of the castor-oil bean, _abrin_, from the jequirity bean, _robin_ from the locust tree, and with the toxins and antitoxins for diphtheria and tetanus. Antitoxins have been prepared experimentally for a large number of both animal and vegetable poisons, including a number for bacterial toxins. The only ones which, as yet, are of much practical importance are _antivenin_ for snake poison, (not a true toxin, however, see p. 275), _antipollenin_ (supposed to be for the toxin of hay fever) and the antitoxins for the true bacterial toxins of _Corynebacterium diphtheriæ_ and _Clostridium tetani_.