Unconjugated (indirect) bilirubin is produced from hemoglobin breakdown in fetal red blood cells, which have a shorter lifespan (80-90 days vs 120 days in adults). The immature neonatal liver has reduced UDP-glucuronosyltransferase activity, limiting conjugation of bilirubin for excretion. When unconjugated bilirubin exceeds albumin binding capacity, it crosses the blood-brain barrier and deposits in the basal ganglia and brainstem nuclei, causing acute bilirubin encephalopathy (kernicterus). Phototherapy converts unconjugated bilirubin into water-soluble photoisomers (lumirubin) that bypass hepatic conjugation and are excreted directly in bile and urine.