EARLY studies of the mechanism of anaphylaxis led to two apparently contradictory views : the so-called ‘humoral theory’, according to which the blood is the site of production of a toxin, the anaphylatoxin1–4; and the ‘cellular theory’, which localizes the anaphylactic reaction in the cells, and assumes that this reaction causes the release of histamine from the tissues5,6. The anaphylatoxin theory was based on the finding that guinea pig serum treated with the specific precipitate or with kaolin, barium sulphate, agar, starch or inulin, becomes toxic for the guinea pig and, if injected intravenously, produces symptoms indistinguishable from true anaphylaxis.