If the phagocytes and similar cells in the blood of a man or animal exposed to the poison produced by localised microbes (such as those of tetanus, diphtheria and septic growths) cannot produce enough antitoxin so as to quickly destroy the poison, we can, and do, nowadays, save his life, by injecting into his blood the required antitoxin, obtained from another animal which we have caused (by injection of the toxin) to produce the antitoxin in excess. That is one sort of "immunity" or "resistance" which we can confer, and is largely in use at the present day--the "antitoxin" treatment. The second poison-repelling chemical activity of the blood, produced by the living cells in and about it, consists in the blood becoming directly poisonous to injurious microbes. It becomes "bactericidal," produces a bactericidal poison (called an alexin) which is usually present in normal blood, but is greatly increased when large numbers of certain poisonous microbes (_e.g._ those of typhoid fever) get into the blood. Again, by other chemical substances produced in it, the blood may, without actually killing the invading bacteria, only paralyse them, and cause them to "agglutinate" (that is, to adhere to one another as an inactive "clot" or "lump"). As the "agglutinating" poison is peculiar (or nearly so) for each kind of microbe, we can tell whether a patient has typhoid by drawing a drop of his blood into a tube, and adding some fresh living typhoid bacilli to it. If the patient had typhoid he will have begun to form the "typhoid-agglutinating" or "typhoid-paralysing" poison in his blood, and the experiment will result in the "agglutination" (sticking together in a lump) of the typhoid bacilli. And so we prove, in a doubtful case, that the patient has typhoid. The third chemical activity of the blood in dealing with poisonous microbes is also one which is conferred upon it by its living cells when excited by the presence of those microbes. It is the production of a "relish" (for so it must be called) which attaches itself to the microbes and renders them attractive to the eater-cells (the phagocytes), so that those swarming amoeba-like floating particles at once proceed to engulf the microbes with avidity. In the absence of the relish (the Greek word for it used by Sir Almroth Wright, its discoverer, is "opsonin"), the eater-cells are sluggish--too sluggish--in their work. They resemble a child who will not eat dry toast, or, at best, only slowly, but will devour rapidly many pieces when the toast is buttered. It is of the utmost importance to us that our white corpuscles, or eater-cells, should not be sluggish but greedy.