Urticaria

A 7-month-old boy is brought to the office because of a recurrent pruritic rash on his body. His parent first noticed a few lesions 4 days ago that disappeared within 2 hours. On subsequent days, similar but widespread lesions occurred that improved within a few hours without any intervention. His parent states he has not had any fevers, fatigue, or loss of appetite recently. He recently started gradually weaning off breast milk exclusively and eats boiled eggs for breakfast. He was born via spontaneous vaginal delivery, and his parent reports he has been healthy with no medical conditions. Vitals are within normal limits. Physical examination shows well-demarcated, raised, erythematous plaques as shown:


Reproduced from: Wikimedia Commons

Which of the following is the most likely pathophysiology of this patient’s condition?