Enzymes are retarded in their action in the first place by their own products, in a very similar manner to bacteria, in the second place the retarding or inhibiting action is brought about by the so-called _anti-bodies_. Of these the longest known and best studied are those which are produced by toxins, and which neutralize the action of the toxins upon the animal organism (anti-toxins).[109] Normal serum also inhibits the action of trypsin and many other enzymes. Another very important group of anti-bodies are the _precipitins_. If the serum of an animal be injected repeatedly into another animal of different species, a precipitin appears in the serum of the animal treated, which causes a precipitate when added to the serum of the first animal. The special importance of this fact is, that it can be utilized as a method of distinguishing between human blood and that of animals, which is often of importance in medical juris-prudence.[110]
This fact has been made use of to distinguish dog dung from other matters, with which it has been adulterated.[111] A perfectly clear extract of dog dung filtered free from bacteria was injected into a rabbit. The serum obtained from the rabbit was found to contain a precipitin, and on addition of the serum to the dog dung extract a precipitate was produced. When the serum was added to the extract from the dung of another animal no precipitate was formed. When added to the extract of the adulterated dung a very much smaller precipitate was produced than with the extract from dog dung alone.
[111] Appelius, Technische Briefe, No. 23, April 1909.
The coli bacteria in the puer also produce an anti-body--_agglutinin_.[112] If a culture of B. coli be examined under the microscope, the bacteria are seen moving here and there in the liquid, evenly distributed. On the addition of a trace of the serum of an animal which has been previously injected with coli bacteria the bacteria on the slide cease their movements and collect together in clumps. They are said to be agglutinated. This property is used to diagnose bacteria in suspected cases of cholera and some other diseases.
The wonderful discoveries that have been made in this direction constitute one of the most marvellous chapters in the history of science. Serum diagnosis and serum therapeutics are now firmly established as invaluable aids to the physician in his fight against disease and death.