The data obtained in this study support the following conclusions:

  1. An abrupt and pronounced reduction of complement follows in 30 to 60 minutes after the reinjection of an antigen into a rabbit. This decrease of complement-activity is probably due to fixation of complement by the complex of extracellular antibody and extracellular antigen in vivo.

  2. A similar mechanism possibly is involved in the loss of complement in anaphylaxis and also in serum sickness.

  3. The antigen-antibody union may also account for the loss of hemolytic complement in the course of various infectious diseases.

  4. The granulocytopenia which generally accompanies the reduction of complement after the reinjection of an antigen appears to be related to anaphylactic and anaphylactoid reactions as well as to an in vivo antigen-antibody reaction. This reaction may however occur in the apparent absence of extracellular antibody.

  5. The complement-reduction reaction is well suited for the study of antigen-antibody and hapten-antibody reactions in vivo. It may be used for the detection of extracellular antigen or antibody. It is highly specific and sensitive, apparently not affected by an excess of antigen or antibody, and may require little antigen for its appearance. At present, however, it is not a quantitative method.

  6. The granulocytopenic reaction may be useful for the early detection of an anaphylactic type of sensitivity to a given antigen.

  7. The granulocytopenic reaction has been observed often in the absence of any cardinal signs of anaphylactic shock.

  8. Further study is required to determine the practical significance of the losses of antibodies, complement, and granulocytes during the early part of the so-called negative phase in immunization.

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Aided by a grant from the Commonwealth Fund.

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