[39] Landois, L., _Zur Lehre von der Bluttransfusion_. Leipzig, 1875.
[40] This is probably true only within the limits of exactness used in these experiments.
[41] Friedenthal, H., "Experimenteller Nachweis der Blutverwandtschaft." _Arch. f. Physiol._, 1900, 494.
This line of investigation had in the meanwhile entered upon a new stage when Kraus, Tchistowitch, and Bordet discovered and developed the precipitin reaction, which consists in the fact that if a foreign serum (or a foreign protein) is introduced into an animal the blood serum of the latter acquired after some time the power of causing a precipitate when mixed with the antigen, _i. e._, with the foreign substance originally introduced into the animal for the purpose of causing the production of antibodies in the latter; while, of course, no such precipitation occurs if the serum of a non-treated rabbit is mixed with the serum of the blood of the foreign species.
In 1897 Kraus discovered that if the filtrates from cultures of bacteria (_e. g._, typhoid bacillus) are mixed with the serum of an animal immunized with the same serum (_e. g._, typhoid serum) it causes a precipitate; and that this precipitin reaction is specific. This fact was confirmed and has been extended by the work of many authors.
Tchistowitch in 1899 observed that the serum of rabbits which had received injections of horse or eel serum caused a precipitate when mixed with the serum of these latter animals.
Bordet found in 1899 that if milk is injected into a rabbit the serum of such a rabbit acquires the power of precipitating casein, and Fish found that this reaction is specific inasmuch as the lactoserum from cow's milk can precipitate only the casein of cow's milk but not that of human or goat milk. Wassermann and Schütze reached the same result independently of each other.
Myers and later Uhlenhuth showed that if white of egg from a hen's egg is injected into a rabbit, precipitins for white of egg are found in the serum of the latter, and Uhlenhuth[42] found, by trying the white of egg of different species of birds, that the precipitin reaction called forth by the blood of the immunized animal is specific, inasmuch as the proteins from a hen's egg will call forth the formation of precipitins in the blood of the rabbit which will precipitate only the white of egg of the hen or of closely related birds.