It has been found that when an immune serum is brought together with a clear filtrate of a bouillon culture of the organism used for injection, there will appear a turbidity which will collect gradually at the bottom of the test-tube as a precipitate.

Such substances are formed whenever foreign albumins, either of vegetable or animal origin, are introduced through parenteral channels. These bodies are called precipitins.

These precipitins are specific in their reaction, and have been used for the purpose of identifying the origin of various albumins. Under the term of the "biologic" blood-test this action has been employed in medicolegal cases to determine the source of blood-stains. It is also made use of in establishing zoölogic relationships between different animals.

If human blood is injected a number of times into a rabbit, the serum from the rabbit's blood will form a precipitate with normal human blood-serum when the two are mixed in a test-tube.

It is thought by some that the agglutinins and precipitins are practically similar substances; agglutination being a bringing together of cells; precipitin action, the bringing together of albuminous particles.

Anaphylaxis is a term applied to an increase of susceptibility to infection; it is the opposite of immunity. It is a reaction that will occur with the parenteral form of injection of foreign proteins of any kind. In order to obtain the characteristic reaction it is necessary that a period of from six to ten days intervene between the first and second injection. A guinea-pig may be sensitized by 0.001 gm. of horse serum introduced into the peritoneal cavity. Eight to ten days later a second injection of 0.1 gm. of the serum is given, at which time the animal will become restless, short of breath, scratch itself violently about the nose, then depressed, and dies within one hour. Autopsy shows the lungs to be greatly distended and numerous small. hemorrhages present. Similar symptoms have been encountered in people who have received antitoxin horse serum. In addition, there are skin eruptions, joint-pains, and edema, a condition known as serum sickness.

The anaphylactic reaction is specific, and the susceptibility, once acquired, may continue throughout the life of the animal, and may be transmitted by the blood of the mother to the offspring. It may be natural or acquired, active or passive. It may also be general or local.

The possibility of local anaphylactic reactions has been made use of in the diagnosis of various diseases, particularly tuberculosis. The subcutaneous injection of tuberculin in a non-tuberculous person will cause no disturbance. The same dose in the tuberculous will cause headache, muscle pains, fever, and local reddening around the site of inoculation. Similar results are obtained if the tuberculin is instilled in the eye (Calmette reaction), but as severe inflammations have been occasioned, the method is not recommended. A like reaction is claimed when luetin, a specially ground-up culture of the Treponema pallidum, is employed as a subcutaneous reaction for syphilis. Much experimentation is being done along this line in respect to the making of diagnoses in various diseases. The symptoms of many diseases may be due to the presence of foreign proteins that have sensitized the individual.

According to Vaughan, anaphylaxis results when the strange protein in the blood reaches the cells and is slowly broken down by enzymic action. The cells, having once acquired the property of destruction, seize eagerly upon the protein the next time it is offered, disintegrate it rapidly, and so disseminate throughout the body the disintegration products, some of which may be toxic and account for the reaction.