Substances cause direct (or complement-mediated in case of anaphylactoid reaction) mast cell activation and subsequent release of histamine not mediated by immunoglobulin.
In contrast to true anaphylactic reactions, no sensitization to allergens is required (i.e., first contact can already lead to anaphylactic shock)
Clinical features: urticaria, pruritus, edema, hypotension, or even symptoms of anaphylactic shock
