Protective Powers of Blood
The normal blood contains substances inimical to bacterial life and is consequently pre-eminent amongst the body defences against disease. It is a commonplace that certain people when exposed to infection do not contract disease. This is due to certain protective qualities of the blood, and such persons are regarded as possessors of a " natural immunity." It is also a familiar fact that one attack of an infectious disease protects against another attack of the same disease. An "acquired immunity" is developed.
This response on the part of the blood can be artificially produced and can be used as a preventive measure against possible infection. By vaccination a mild attack of cow-pox (believed to be an attenuated variety of smallpox) may be induced. The natural protective powers of the blood are stimulated and a relative immunity to smallpox is attained. Vaccine treatment is simply the introduction into the body of dead or attenuated living infectious material with the object of increasing the resistive powers of the blood. A vaccine may be used either to prevent infection as in enteric or cholera or to aid in cure as in rabies or catarrhal inflammations.
When the actual poison of bacteria is injected into the body it stimulates the production of a natural antidote or antibody. A minute dose of the toxin of diphtheria introduced at intervals into the blood-stream of an animal results in the formation of a considerable quantity of antitoxin. Such an animal is now able to survive a dose of toxin many times larger than that which previously would have killed it. An immunity to the toxin has resulted. The antitoxin neutralizes the toxin in much the same way as an acid neutralises an alkali and their mixture in equal quantities is harmless. Blood serum, rich in antitoxin, can be procured by the gradual immunization of an animal (usually the horse) and this serum can be used to combat a toxaemia in human beings. This is known as " passive immunisation" to distinguish it from the " active immunisation " of vaccine inoculation. Passive immunisation by the injection of antitoxin is universally adopted in the treatment of diphtheria and tetanus.
The blood-plasma possesses natural protective powers in the form of "bacteriolysins", substances antagonistic to organismal life. Also, invasion by bacteria stimulates the " agglutinating power " of the plasma. The invaders are rendered immobile and are clumped together so that their activities are hindered.
It is the function of the phagocytes (white blood cells) to eat up invading organisms, but they appear unable to do so unless there are present in the plasma substances called "opsonins " which prepare the organisms for destruction. The activities of the phagocytes, upon which the power of resistance depends, varies with the quantity of opsonins, i.e. the opsonic index.
The means of combating bacterial foes are many. They may be killed by bacteriolysins; rendered immobile by agglutinins; eaten up by phagocytes aided by opsonins, or the toxins may be neutralised by antitoxins. These are the main methods by which the body is enabled to resist or to recover from diseases of organismal origin.