We may now consider shortly how these new facts were received and what theories of explanation were put forward to explain continued insusceptibility to disease. It had of course been known for a long time past that one attack of small-pox, for example, in some degree protected the individual from a subsequent attack of the same disease. To that experience it was now necessary to add a large mass of experimental evidence with regard to toxins and antitoxins. The theories of immunity were as follows:
1. _The Exhaustion Theory._ The supporters of this idea argued that bacteria of disease circulating in the body exhausted the body of the supply of some substance or condition necessary for the growth and development of their own species.
2. _The Retention Theory._ It was surmised that there were certain products of micro-organisms of disease retained in the body after an attack which acted antagonistically to the further growth in the body of that same species.
3. _The Acquired Tolerance Theory._ Some have advanced the theory that, after a certain time, the human tissues acquired such a degree of tolerance to the specific bacteria or their specific products that no result followed their action in the body. The tissues become acclimatised to the disease.
4. _The Phagocyte Theory._ This theory, which gained so many adherents when first promulgated by Metschnikoff, attributes to certain cells in the tissues the powers of "scavenging," overtaking germs of disease, and absorbing them into their own protoplasm. This, indeed, may be actually witnessed, and had been observed before the time of Metschnikoff. But it was he who applied it to disease. He came to the conclusion that the successful resistance which an animal offered to bacteria depended upon the activity of these scavenging cells, or _phagocytes_. These cells are derived from various cellular elements normally present in the body: leucocytes, endothelial cells, connective-tissue corpuscles, and any and all cells in the body which possess the power of ingesting bacteria. If they are present in large numbers and active, the animal is insusceptible to certain diseases; if they are few and inactive, the animal is susceptible.