Protective Antibodies Following Oral Administration of Pneumococcus Types 2 and 3 to Rats, with Some Data for Types 4, 5 and 6
Authors: Ross Victor 
Published: January 2023

Summary
No circulating antibody was found in the blood of white rats against those types of pneumococci to which both very young and adult individuals are susceptible (types 1 and 5).
Antibody is present in the blood of many, although not all of the rats, for those types of pneumococci against which both very young and adult individuals have a high degree of resistance (types 4 and 6). The concentration is very low.
Young rats available in the present work had a high degree of resistance to type 2, and the sera of about half of all the animals contained small amounts of protective substance. Adult rats had moderate quantities. Young rats have on a previous occasion been found to succumb to very small doses of type 2 pneumococcus, and it is reasonable to suppose that the blood of these animals possessed no protective power.
Most young rats (about 25 to 75 grams) succumb to an injection of type 3 pneumococcus, and no antibody is found in the blood of the susceptible animals. The larger rats become resistant and develop antibodies.
The artificial immunity produced against type 3 by feeding the vaccine is accompanied by the appearance of circulating antibodies. This has previously been demonstrated for type 1. The results of the experiments with type 2, although not so definite as those for types 1 and 3, nevertheless indicate that the blood acquires protective power when the vaccine is fed. On account of the high natural resistance of the available rats, it was impossible to demonstrate an artificial immunity and circulating antibody in the same animal.
The data show a higher concentration of protective antibody in the blood of rats for type 4 pneumococcus than in that of controls. We assume, by analogy with types 1, 2, and 3, that an increase in resistance takes place simultaneously, and that it is masked by the high natural immunity.
There is no evidence that feeding type 6 produces any change in the protective value of rats' sera. It is worth recalling that feeding type 2 vaccine to humans tends to elicit small amounts of type 6 antibody.
The increased resistance against type 5 produced by feeding the vaccine is so little in amount that the failure to detect accompanying circulating antibody is not surprising. It seems curious, in view of the closer serological relationship existing between types 2 and 5 than between type 2 and types 1 and 3 (as evidenced by the agglutination reaction), that the results of oral immunization against the last two named organisms should be more like those obtained with type 2 than are those which follow feeding type 5. As we have shown that feeding the pneumococcus polysaccharide (types 1 and 3) confers an immunity on rats, and since it is a reaction between this constituent of the organism and antibody at the surface of the cell which is evidenced by agglutination (3), it would seem that the particular chemical grouping in the “soluble specific substance” molecule taking part in the agglutination reaction is different from the grouping which actually elicits agglutinin formation, or immunity.9