Antibacterial systems of serum in relation to nonspecific immunity to infection.

By nonspecific resistance to infection we presumably include those factors which determine why some animal species resist infections to which others readily succumb. These factors do not appear to be identifiable with specific antibody since there may be widely differing responses to two organisms which are antigenically almost identical. A case in point is the susceptibility of normal mice to Salmonella typhimurium and their native immunity to Salmonella gallinarum or Salmonella schottmuelleri, to be contrasted with the susceptibility of chickens to S. gallinarum and resistance to the other two bacterial strains. Because of the similarity in antigenic structure of these three Salmonella species it seems unlikely that the immunity of normal mice to S. gallinarum is due to a specific normal antibody which is directed successfully against this strain, but is ineffective against S. typhimurium. There are many other examples like this. The contribution, if any, serum factors may make in determining this natural species immunity will be discussed later. When the experimental infections most commonly studied in the laboratory are considered it is apparent that the great majority of them are initiated through a hypodermic needle for convenience. This must not be forgotton when examining the problems of natural susceptibility. For example, the factors responsible for the native resistance of mice to Diplococcus pneumoniae are unlikely to be found in mouse serum, since although natural pneumococcal infection rarely if ever occurs in mice, these bacteria are extremely lethal when introduced artificially. Therefore, resistance is likely to be due to factors encountered before the organisms reach the blood stream. A few moments spent in clarifying terms may be helpful in getting these problems into focus. If the pathogenicity of an organism for a certain animals is defined as its ability to cause disease naturally in that animal, then this can be put into an equation as follows: