1. The A carbohydrate isolated from Type I pneumococcus by Pappenheimer and Enders, on the basis of elementary analysis, the presence of the acetyl group and its immunological properties, appears to be identical with the acetyl polysaccharide described by Avery and Goebel.

2. The A carbohydrate possesses a greater anti-opsonic action than either the deacetylated substance obtained by boiling in alkali or the soluble specific substance of Type I pneumococcus prepared according to the procedure of Heidelberger, Goebel and Avery. The opsonic titre of normal human serum is practically eliminated upon the addition of the A carbohydrate—an effect not observed with equivalent amounts of either the deacetylated material or the specific soluble substance. In immune serum, the A carbohydrate brings about a quantitatively greater reduction in opsonic activity than its derivatives, but it has not been possible to demonstrate complete inhibition of phagocytic action by the method of absorption of antibody.

3. In a system of normal human serum and leucocytes capable of destroying Type I pneumococcus, the bactericidal effect maybe entirely removed upon the addition of the A carbohydrate. It proved impossible to inactivate the bactericidal action with the deacetylated substance in equivalent proportions. In this system, the A carbohydrate was about 64 times more effective as an antibactericidal agent than the deacetylated compound. Essentially similar results were obtained in a study of the antibactericidal properties of the A carbohydrate and the deacetylated derivative in the presence of anti-Type I pneumococcus rabbit serum added to a mixture of exudative leucocytes and the defibrinated blood of the rabbit.

4. The mouse-protective titre of anti-Type I pneumococcus rabbit serum is lowered to a greater degree after absorption with the A carbohydrate than it is by similar treatment with the deacetylated compound. Absorption with the A carbohydrate does not, however, completely remove the protective antibody.

5. As Avery and Goebel have shown in the case of the acetyl polysaccharide, so the A carbohydrate, when administered in very small quantities, protects mice against an otherwise fatal dose of Type I pneumococcus. Active immunity in mice has been obtained with as little as 0.00005 mg. of the A carbohydrate administered in a single dose. Doses larger than 0.005 mg. confer no protection on these animals. Deacetylization of the A carbohydrate after boiling in N/10 sodium hydroxide destroys its protective capacity while similar treatment in N/50 alkali does not completely remove its immunizing property. Active immunity may arise within 3 days following a single injection of the A substance. It appears to be at its height from 6 to 25 days thereafter, and is retrogressive by the 49th day following vaccination. Injection of the A carbohydrate into immunized mice immediately before infection deletes the state of resistance.

6. The immunity actively induced as a result of injection of the A carbohydrate may be passively transferred to normal mice with the serum of vaccinated animals.

7. Since the evidence obtained in the course of this study indicates that the A carbohydrate of Type I pneumococcus and the acetyl polysaccharide of Avery and Goebel represent the same chemical substance, it is suggested that the designation "A carbohydrate" or "A substance" be relinquished in favor of the more exactly descriptive term "acetyl polysaccharide."

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