Exposure to allergen → Anaphylaxis (phase 1) → Patient self-administers Epinephrine autoinjector → Symptoms resolve → in about 2 hrs to 3 days → another anaphylactic reaction (phase 2) without a second exposure to the allergen. That is a biphasic anaphylactic reaction.
A biphasic reaction is a two-phase anaphylactic reaction. “This means that after anaphylaxis is treated and the symptoms go away, they return without you being re-exposed to the allergen. The second reaction can be less severe, equal to or more severe than the first reaction. This makes them dangerous as some people may think that they are fully recovered and they may not have their adrenaline auto-injector as they will have used it to treat their first reaction. A second reaction can occur as little as 2 hours and as much as 72 hours after the first reaction. The average length between reactions is 6-10 hours.” https://www.allergylifestyle.com/biphasic-reaction/
Further Reading / References
Ann Allergy Asthma Immunol. 2015 Sep;115(3):217-223.e2. Epidemiology and clinical predictors of biphasic reactions in children with anaphylaxis. https://www.ncbi.nlm.nih.gov/pubmed/26112147
Ann Allergy Asthma Immunol. 2005 Sep;95(3):217-26; Biphasic anaphylactic reactions.
https://www.allergylifestyle.com/biphasic-reaction/