CH. XXIX.] IMMUNITY 473 



disease. Vaccination produces in a patient an attack of cowpox or 

 vaccinia. This disease is either closely related to smallpox, or 

 maybe it is smallpox modified and rendered less malignant by passing 

 through the body of a calf. At any rate, an attack of vaccinia renders 

 a person immune to smallpox, or variola, for a certain number of 

 years. Vaccination is an instance of what is called protective inoculation, 

 which is now practised with more or less success in reference to other 

 diseases, such as plague and typhoid fever. The study of immunity 

 has also rendered possible what may be called curative inoculation, or 

 the injection of antitoxic material as a cure for diphtheria, tetanus, 

 snake-poisoning, etc. 



The leucocytes or phagocytes destroy bacteria by feeding on 

 them ; but the fluid part of the blood is often antagonistic to bacterial 

 life, and this power was first discovered when the effort was made to 

 grow various kinds of bacteria in it ; it was looked upon as probable 

 that blood-serum would prove a suitable soil or medium for this 

 purpose. It was found in some instances to have exactly the 

 opposite effect. The chemical characters of the substances which kill 

 the bacteria are not fully known ; indeed, the same is true for most 

 of the substances we have to speak of in this connection. Absence 

 of knowledge on this particular point has not, however, prevented 

 important practical discoveries from being made. 



So far as is known at present, the substances in question are 

 protein in nature. The bactericidal powers of blood are destroyed by 

 heating it for an hour to 55 C. Whether the substances are derived 

 from the leucocytes is a disputed point. The substances, whatever 

 be their source or their chemical nature, are called lacterio-lysins. 



Closely allied to the bactericidal power of blood, or blood-serum, 

 is its globulicidal power. By this one means that the blood-serum of 

 one animal has the power of dissolving the red blood-corpuscles of 

 another species. If the serum of one animal is injected into the 

 blood-stream of an animal of another species, the result is a destruction 

 of its red corpuscles, which may be so excessive as to lead to the 

 passing of the liberated haemoglobin into the urine (haemoglobinuria). 

 The substance or substances in the serum that possess this property 

 are called hcemolysins, and though there is some doubt whether 

 bacterio-lysins and haemolysins are absolutely identical, there is no 

 doubt that they are closely related substances. 



Normal blood possesses a certain amount of substances which are 

 inimical to the life of our bacterial foes. But suppose a person gets 

 run down ; every one knows he is then liable to " catch anything." 

 This coincides with a diminution in the bactericidal power of his 

 blood. But even a perfectly healthy person has not an unlimited 

 supply of bacterio-lysin, and if the bacteria are sufficiently numerous 

 he will fall a victim to the disease they produce. Here, however, 



