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. It appears that the bacteria or other foreign bodies in the blood which are attacked by the phagocyte become assimilated until they are a part of the phagocyte itself. Metschnikoff explained also how it comes to pass that the phagocyte is able to encounter bacteria when both are circulating through the blood. It is guided in this attack upon the organisms by a power termed _chemiotaxis_. The bacteria elaborate a chemical substance which attracts the phagocyte, and this is termed "positive chemiotaxis."[80] But it may occur that the chemical substance produced by the bacteria may have an opposite, or repellent, effect upon the leucocytes, in which case we have "negative chemiotaxis."[81] It is not to be wondered at that such a theory of immunity based upon microscopical observations, should at first have been widely accepted, and there can be no doubt that Metschnikoff has collected a considerable mass of evidence in support of a theory of phagocytosis. But when it came to be known that blood serum, from which all leucocytes (phagocytes) had been removed, possessed the same immunising effect as before, it was clear that such effect was a property of the serum _per se_, and not only or wholly due to the scavenging power of certain cells in it. Even the phagocyte theory depends largely for its validity upon chemiotaxis, which latter was a property of the products of the bacteria contained in the blood serum.