There are several different types of reaction which could occur:
Uniphasic – fast onset and rapidly progressive symptoms, but once treated, the symptoms go and don’t return.
Bi-phasic – these are reactions which may be mild or severe to start with, followed by a symptom-free period, and then increasing symptoms with respiratory and circulatory problems, which could occur some hours later. The exact incidence of bi-phasic reactions are unknown, although studies suggest 1-20% incidence. There is no reliable way of predicting who will have a bi-phasic reaction.
Protracted anaphylaxis – this can last for several days and the patient may need inotrophic and ventilatory support.