A study on hemagglutination and hemolysis by Escherichia coli O111 and O55 (rabbit) antisera and on hemagglutination and hemolysis inhibition by E. coli O111 and O55 antigens revealed the following facts.

1. Red blood cells of man, dog, rabbit, guinea pig, sheep, rat, and chicken adsorb E. coli O111 and O55 antigens and thus become specifically agglutinable by the homologous E. coli antisera.

2. The adsorption of these E. coli antigens is a function of the concentration of the antigen, the time (from 5 minutes to 2 hours) of treatment of the red blood cells with the antigen, and the concentration of the red blood cells used.

3. Red blood cells of man and sheep adsorb simultaneously both antigens, as indicated by the fact that both antisera give agglutination of all red blood cells. Complete agglutination does not occur when a mixture of red blood cells treated separately with the two antigens is added to one or the other of the two antisera.

4. Treatment of red blood cells of man with one of the antigens does not block the adsorption of the second antigen. Human cells treated with either or both antigens are still agglutinated by the homologous blood group (A, B, and Rh)-specific antibodies.

5. In the presence of guinea pig complement, E. coli O111 and O55 antisera produce hemolysis of modified human red blood cells in titers of the same order of magnitude as those giving hemagglutination and bacterial agglutination. The same antisera produce hemolysis of sheep cells treated with the identical antigens in titers exceeding by far those giving agglutination of modified human or sheep red blood cells.

6. Both sediment and supernate of a boiled E. coli suspension are capable of modifying red blood cells for E. coli hemagglutination; in contrast, the supernate obtained from an unboiled suspension and then heated does not modify red blood cells for hemagglutination, although it contains the antigen which can specifically adsorb E. coli antibodies, as shown by means of the hemagglutination and hemolysis inhibition tests.

7. Both the unheated and the boiled suspensions of E. coli O111 and O55 inhibit hemagglutination and hemolysis specifically.

8. Rabbit red blood cells modified by either E. coli O111 or 055 antigens, upon intravenous injection into rabbits, engender specific E. coli antibodies.

The significance of the results is discussed.

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