disease and subsequently have had their immunity intensified by injections of highly virulent blood, the cattle so vaccinated will develop rinderpest in a mild form and will subsequently on recovery be also immune; and yet during the process of immunization their blood contains highly virulent parasites so that if a little of it be introduced into non-protected and healthy cattle, they will be given rinderpest and will die of it.
The reaction of the body to the bacterial vaccines injected is out of proportion to the quantity of culture introduced. Thus two milligrams of dead cholera bacilli injected under the skin of human beings will yield enough of the specific immunity substance for these bacilli to bring about the destruction of 60,000 or more milligrams of the culture. There can be, therefore, no direct transformation of the cholera bacilli into immunity bodies, but they must exert a stimulus on certain cell functions through which the immunity. principles are produced; and the quantity of their formation depends not on the weight of crude bacilli introduced, but on the strength of the stimulus impressed upon the sensitive cells to which they react in a specific and remarkable manner.
Is it possible in the course of an established infection to reinforce the resistance of the body? I have already stated that it is not practicable to bring out at the height of an infection an efficient heightened reaction of physiological resistance; but from this it does not follow that under these conditions a special form of immunity reaction may not be elicited. The tuberculin reaction, or that part of it which is specific, may be cited as an example of this kind of reinforcement; and whatever there is of value in the treatment of infectious diseases by means of dead cultures of their specific bacteria—"vaccines" so-called—must be of the nature of an intensified immunity reaction. What is sought to be accomplished in the latter case is the formation in certain uninfected localities—in the subcutaneous tissues, for example—of immunity principles that afterwards by escaping into the blood shall assist in the termination of an infectious process situated elsewhere in the body. Such local foci of immunity as it is designed to create in the subcutaneous tissue are not unknown. The pleura can be given a local immunity to the typhoid bacilli; the subcutaneous tissue to tetanus toxin, and it is highly probable that the normal resistances exhibited by our mucous membranes to the pathogenic bacteria they harbor are examples of such local immunities.
I fear that I have carried you far afield and into somewhat devious paths of immunity to disease. You will, I know, not complain and hold it to the detriment of medical science, that these paths have not been already converted into fine open roads. But you will prefer to recall how brief is the time since where the paths now are there were only wood and tangle.