Summary of Immunity as Applied to Protection from Disease.
The discussion of “immunity problems” in the preceding chapters serves to show that protection from disease either as a condition natural to the animal or as an acquired state is dependent on certain properties of its body cells or fluids, or both. The actual factors so far as at present known may be summarized as follows:
1. Antitoxins which neutralize true toxins; shown to exist for very few diseases.
2. Cytolytic substances which destroy the invading organism: in reality two substances; amboceptor, which is specific, and complement, the real dissolving enzyme.
3. Phagocytosis or the destruction of the invading organisms within the leukocytes.
4. Opsonins which render the bacteria more readily taken up by the phagocytes.
5. Enzymes other than complement possibly play a part in the destruction of some pathogenic organisms or their products. This remains to be more definitely established.
6. It is possible that in natural immunity there might be no receptors in the body cells to take up the organisms or their products, or the receptors might be present in certain cells but of a very low chemical affinity, so that combination does not occur. It is even highly probable that many substances formed by invading organisms which might injure specialized cells, such as those of glandular, nervous or muscle tissue, have a more rapid rate of reaction with, or a stronger affinity for, lower unspecialized cells, such as connective and lymphoid tissue, and unite with these so that their effects are not noticed.
The importance of these different, factors varies in different diseases and need not be considered in this connection.
The question “which of the body cells are engaged in the production of antibodies” is not uncommonly asked. On physiological grounds it would not seem reasonable that the highly specialized tissues above mentioned could take up this work, even though they are the ones which suffer the greatest injury in disease. Hence it is to be expected that the lower or unspecialized cells are the source, and it has been shown that the antibodies are produced by the phagocytes (though not entirely as Metchnikoff maintained), by lymphoid tissue generally, by the bone marrow and also by connective-tissue cells, though in varying degrees.