1. Immunity cannot be completely explained by antigen-antibody reactions, even if the term ‘antibodies’ be made sufficiently elastic to include various obscure properties which are exhibited, in vivo, in the actively immune animal. Various other factors have to be considered. One of these is the influence of stimuli upon the vital capacities of bacteria.
2. Transmissible bacterial autolysis appears to be due to a stimulus acting upon the growing bacterial cell and leading to the splitting off of a certain number of daughter-cells which are non-viable, and consequently undergo autolysis.
3. Transmissible autolysis is not due to a stimulus sui generis but is no more than a particular incident in the general phenomena of bacterial variation.
4. The secretions of bacteria in pure culture may stimulate, control, or retard their growth and may lead to the production of variants.
5. When introduced into the animal body, bacteria encounter stimulants of animal origin which may be either favourable or unfavourable to their growth and are to be distinguished from the stimulants attributable to the bacteria themselves.
6. One aspect of the differences between natural immunity and natural susceptibility may be interpreted as due to differences in the stimuli inherent in the particular animal species and to consequent differences in their effects upon the particular bacterial species.
7. Similarly, when no better explanation is available, the acquired immunity (active or passive) of a susceptible animal may be interpreted as a change of the animal's stimulant action from one which was favourable (or indifferent) to the growth of a bacterium to one which is adverse, i.e., a stimulant to the reproduction of daughter-cells which are non-viable in the animal body.
8. Leucocytes are one of the sources of material possessing two kinds of properties, viz., (a) stimulant action on the growth capacities of cells (both bacterial and animal) and (b) enzyme action on the constituents which living and dead cells possess in common. The older researches on the characters of leucocytic extracts were occupied with (b), though they may occasionally be linked up with (a), since there are some indications that their leucocytic material was also acting as „lytic substance.”
9. In some cases it must remain, for the present, an open question whether demonstrable antibacterial action is attributable to some more or less obscure enzymes or to what I have termed „stimuli.”
10. A stimulus may be a substance which is also an antibody, and its stimulative properties may be highly specific. But it would be absurd to assume that for each special kind of stimulative effect there is a special and chemically distinctive kind of stimulus. Both stimuli and antibodies usually possess a wide range of different combining affinities which cannot be explained on the „mosaic” theory that each different combination is due to the presence (in the stimulus or antibody) of a different chemical group.
11. A stimulus, as distinct from a food, causes the bacterial cell to function in a particular way but is not incorporated as part of the structure of the cell. This convenient distinction, however, does not imply that there is necessarily a sharp line of demarcation between a stimulus and a food.
