Interaction in free solution of highly purified preparations of human C'1 esterase, C'4, C'2, and C'3, in the presence of Mg2+, resulted in rapid generation of an activity indistinguishable by biological criteria from anaphylatoxin. The formation of anaphylatoxin was associated with immunoelectrophoretic conversion of C'3 to anodically faster migrating proteins and was unaffected by the presence or absence of added C'5.

The biological properties of human anaphylatoxin prepared in this manner include: contraction and desensitization of isolated guinea pig ileum, failure to contract isolated rat uterus, enhancement of vascular permeability in guinea pig skin, degranulation of mast cells in guinea pig mesentery preparations, and liberation of histamine from suspensions of rat peritoneal mast cells. The smooth muscle-contracting and permeability enhancing properties were fully blocked by an antihistaminic drug, triprolidine. No cross-desensitizing activity on guinea pig ileum was demonstrable between rat and human or guinea pig and human anaphylatoxins but a closer biological relationship between rat and guinea pig anaphylatoxins was observed.

It is concluded that anaphylatoxin is a product of the complement system. Its possible relationship to apparently similar activities currently being obtained in other laboratories has been discussed.

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