X-radiation and nitrogen mustard administration inhibit the formation of precipitins for whole bovine serum and bovine serum gamma globulin in the rabbit.

When specific antibody formation is inhibited by these agents, intravenous injection of a single large dose of bovine serum gamma globulin is not usually followed by the development of tissue lesions 9 to 11 days later, as occurs fairly regularly in control animals.

A fall in titre of serum complement to very low levels for 3 to 5 days is closely correlated in time with the disappearance of antigen from the circulation following the intravenous injection of single large doses of bovine serum albumin and bovine serum gamma globulin. A rise in complement titre to normal levels occurs as antibodies appear in the serum.

This sudden fall in complement titre is correlated with the development of characteristic lesions, and does not occur when antibody formation is inhibited.

The data presented are interpreted as evidence in favor of the concept that the lesions are due to a reaction between antigen fixed in or on tissue cells and circulating antibody.

The possible significance of serum complement in the pathogenesis of anaphylactic tissue lesions is discussed.

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