An anaphylactic reaction: This involves the liberation of histamine, 5-hydroxytryptamine (5-HT) and associated vasoactive substances from mast cells and basophils, following exposure to a foreign substance (the drug) to which the individual has become sensitized. It is usually an IgE-mediated reaction.
An anaphylactoid reaction: This is clinically indistinguishable from the above, but the release of histamine, etc. from mast cells is triggered by non-IgE mechanisms.
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