A Rare Case of Cow's Milk Protein Allergy in a 12-Hour Old Neonate With Rectal Bleeding

Hematochezia presenting in the neonatal period requires a thorough exploration of life-threatening and benign pathologies, including food protein-induced allergic proctocolitis (FPIES). This non-IgE-mediated food allergy is most commonly associated with cow's milk protein (CMP). It classically presents in the first weeks to months of life, following the introduction of a cow's milk-based formula or breast milk where the maternal diet includes cow's milk. We present a rare case of extremely early onset of CMP-induced proctocolitis, in a neonate presenting at 12 h of life with hematochezia. How does a food allergy develop at such a young age? We explore the current theories explaining this phenomenon.