This deduction is also supported by the antitoxic immunity which may be conferred on _unicellular_ beings in which the cell alone enters into play.
Phagocytes no doubt manufacture many other soluble ferments corresponding with the elements which they absorb, for, in a vaccinated organism, divers new specific properties of the serum are to be found, such as that of agglutination, precipitation, etc. Humoral properties may be more or less durable, in proportion as the products manufactured by the phagocytes are more or less rapidly evacuated by the organism.
All these humoral properties, traced back to their first source, depend upon the digestive activity of the phagocytes, since they are the products of that digestion. In cases where it has not yet been possible to make a direct demonstration of this, it becomes evident through analogy and experiments pointing in that direction.
To sum up, according to Metchnikoff, "_Immunity in infectious diseases is linked with cellular physiology, namely, with the phenomenon of the resorption of morbid agents through intracellular digestion._ In a final analysis, the latter (as also the digestion of food in the gastro-intestinal tube) reduces itself to phenomena of a physico-chemical order; however, it is a real _digestion_ accomplished by the living cell.... The study of Immunity, from a general point of view, belongs to the subject of Digestion."
Immunity against diseases is but one of the manifestations of an immunity on a much larger scale, always based, in final analysis, on the sensitiveness of the living cellular protoplasm. The sensitiveness of the nervous cells extends this phenomenon to the psychical domain. They also are capable of becoming accustomed to external irritations of all kinds, hence constituting a psychical immunity for the organism. We all know that one can become accustomed to many painful or violent sensations; and, as Metchnikoff says: "... It is very probable that the whole gamut of Habit, starting from the unicellular beings, who accustom themselves to live in an unsuitable medium, to cultured men who acquire the habit of not believing in human justice, rests on one and the same fundamental property of living matter."