HAEMOLYSIS AND IMMUNITY 111 



Precipitins. These bodies are present in the blood of an immunized animal, 

 and produce a precipitation of the soluble bacterial proteins if added to a filtrate of 

 the culture used for immunizing the animal. Their action is specific. Precipitins 

 can be produced by the injection of any protein, provided that the protein is 

 foreign. It is useless to try and immunize a rabbit against rabbit's serum, and 

 it is better not to employ closely related specie?, such as rabbit and guinea-pig. As 

 the result of the injection of horse's serum into a rabbit a precipitin is obtained in 

 the rabbit's serum Avhich precipitates the proteins in the serum of the horse and of 

 no other animal. Similarly, as the result of the injection of cow's milk a precipitin 

 is obtained which precipitates only the proteins of cow's milk and not those of the 

 milk of any other animal. The specific action of precipitins shows us that the structure 

 of the homologous proteins varies in different animals. Only by the injection of 

 foreign proteins can precipitins be produced, and the power of the proteins to 

 produce precipitins is lost when they become split up into peptones. Fats and 

 carbohydrates cannot produce precipitins. The precipitation test can only be made 

 outside the body. If serum containing precipitins be injected intravenously, it does 

 not cause precipitation, but provokes an increase in the number of leucocytes. The 

 material obtained from a mummy five thousand years old gave the precipitin 

 reaction for man. 



The action of precipitins is modified by the concentration of electrolytes. A 

 precipitin has two linkages: one the haptophor, which links on to the protein, and 

 another linkage (destroyed by heating to 60 C.), which induces the change bringing 

 about the precipitation of the protein. 



The precipitin appears in the blood about six days after the first injection of 

 protein has been made. Following each subsequent injection it disappears for a time, 

 and then appears again. When the injections are finished, the precipitin quickly 

 disappears from the blood, its fate is not known, and it cannot be detected in the urine. 

 The source of precipitins is not known. As an increased number of leucocytes 

 (leucocytosis) follows each injection, it has been thought that these produce the 

 precipitins. 



The precipitins are attached to the globulins in the plasma and cannot be separated 

 from them. 



Cytotoxins. By the injection of animals' cells, bodies called cytotoxins are pro- 

 duced in the blood. These are capable of destroying the foreign cells injected. Red 

 corpuscles, leucocytes, spermatozoa, kidney substance, stomach, thyroid, and nervous 

 tissues, have all yielded specific cytotoxins, and so-called erythrolytic, nephrolytic, 

 and other " lytic " sera have been produced. Small gastric ulcers have been caused 

 by injecting the blood of one animal immunized against the injections of the mucous 

 membrane of the stomach of another species of animal. The red corpuscles afford 

 the best material for studying this phenomenon (c/. Haemolysis, p. 105). 



Hypersusceptibility, Anaphylaxis. Some people are extraordinarily sensitive 

 to the ingestion of certain nutritive material such as crab flesh, strawberries, egg white. 

 They are made sick by eating one or other of these things, or suffer from the eruption 

 of a nettle-rash, the result of a disturbance of the equilibrium between the osmotic 

 pressure of the tissues and tissue lymph, which in its turn is due to the toxic effect 

 which the ingested material has on the tissue metabolism. Similarity, sensitivity 

 may be produced by injection of a small dose of a foreign protein e.g., of horse serum ; 

 the sensitivity is so increased that a second dose of the same serum, containing perhaps 

 little more than a millionth of a gramme of protein, may produce the severest symptoms 

 of intoxication and even death. The hypersusceptibility or hypersensitivity thus 

 induced is termed anaphylaxis. The initial cause of the symptoms seems to be con- 

 striction of the bronchial tubes and obstruction of the airway and a great fall in the 

 blood -pressure, accompanied by congestion and even haemorrhages in the mucous 

 membranes of the bowels. Convulsions, follow the consequent anaemia of the brain. 

 If the animal recover, it is immune to further injections of this serum. The sensi- 

 tivity lasts a very long time. Anaphylaxis has been the cause of alarming symptoms 

 in man in certain cases where a second dose of antitoxic serum has been given after 

 an interval of time. (In some 10 per cent, of normal individuals a single injection 

 of antitoxic serum is followed by similar though less severe symptoms.) Anaphy- 

 . laxis may be regarded as the opposite to immunity. 



