1. Expanded data have been presented and analyzed concerning interference of ABO incompatibility with formation of Rh antibodies in Rh-negative men injected with Rh-positive blood. ABO-incompatible Rh-positive blood appeared to be also less capable of stimulating secondary responses to Rh antigen.

  2. Forty per cent of the men who had failed to form Rh antibodies after injection with ABO-incompatible blood did form Rh antibodies after subsequent injection of ABO-compatible blood.

  3. Injection into A Rh-negative recipients of A Rh-positive blood coated in vitro with anti-A was an efficient stimulus for formation of Rh antibodies.

  4. Injection into O Rh-negative men of O Rh-positive blood coated in vitro with Rh antibodies was completely ineffective in inducing Rh antibodies. Subsequent injection of such recipients with noncoated Rh-positive blood led to the development of Rh antibodies in 50%.

  5. These data were interpreted as compatible with the hypothesis of “clonal competition for antigen,” namely, that presence of large numbers of antibody-forming cells for one red cell factor may interfere with antibody response to another blood factor contained in the same red cell.

  6. On the basis of preliminary experiments the possibility was considered that, even without preformed antibody in the host, presence of more than one difference in blood factors between recipient and donor may lessen likelihood for isoimmunization in certain circumstances.

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