Biphasic anaphylactic reaction

What it is

A biphasic reaction is the recurrence of symptoms that develop after the apparent resolution of the initial anaphylactic reaction, without additional exposure. No new exposure is needed.

It occurs in 5 to 20% of adults and in 6% of children. It usually occurs within 10 hours of the initial reaction, but has been reported to occur up to 72 hours later.

The reaction it follows is anaphylaxis, a type 1 hypersensitivity that is mostly histamine mediated, with histamine causing vasodilation and increased vascular permeability, prostaglandin causing bronchoconstriction, leukotrienes causing vasodilation, proteases such as tryptase causing bronchoconstriction, and proteoglycans such as heparin activating coagulation and DIC. An anaphylactoid reaction is bradykinin mediated, for example a CT contrast reaction; it needs no prior sensitization, it can occur on first exposure, it takes usually more exposure than anaphylaxis to elicit the reaction, and it is not predicted by allergy skin tests, while an anaphylactic reaction requires prior sensitization or cross-reactivity, is elicited by very little exposure, and is predicted by them.