It is well known that if the yeast plant (which is very similar in many details to bacteria) is grown in a solution of sugar, the sugar is broken up and disappears, and two new substances formed from the sugar take its place. These are alcohol and carbonic acid gas. If bacteria grow in the blood, they do not produce alcohol, but they do produce other poisons in a way analogous to that by which yeast produces alcohol. These poisons are called _toxins_. There are substances in the fluid part of the blood which are called antitoxins, because they neutralise the toxins produced by the bacteria. Their presence constitutes a means of defence against the harmful effects the toxins would otherwise produce. The marvellous part of the defence is that, although we all have a certain amount of antitoxin in our blood, the amount increases in proportion to the amount of toxin. It is a familiar fact that rough manual labour increases the hardness of the hands; friction stimulates the epidermis or outer skin, so that it grows in thickness. The body affords numerous similar instances of how it is capable of rising to the occasion and increasing its defences. Just in the same way, the presence of a toxin stimulates the living cells to produce more and more antitoxin, and the blood remains rich in the antitoxin for a considerable time afterwards. This explains why a person who has had an infectious disease does not take it readily a second time; he is immune for a certain number of years, because his blood is so rich in the antidote. Now, the principle of serum treatment depends on those ascertained and definitely proved facts. In the modern treatment of tuberculosis, for example, the aim of the physician is to increase nature's method of cure: good food and pure air do much to increase the healthiness of the blood and fortify its natural power, of destroying the germs; sometimes this alone suffices. At other times it is not sufficient, particularly if the disease has advanced and the number of bacteria is too great for the enfeebled white corpuscles to deal with. Then the physician goes a step farther, and administers the appropriate opsonin by injecting it under the skin, again simply increasing the resistance of his patient by a perfectly natural method.