a. 2, 3 BPG in blood(Ref: Nelson's 20/e p 2336, Ghai 8/e p 345)Greater affinity of fetal Hb for oxygen is explained by lack of fetal Hb's interaction with 2,3-bisphosphoglycerate (2,3-BPG or 2,3-DPG); In adult red blood cells, 2,3 DPG decreases the affinity of hemoglobin for oxygen2,3-BPG interacts less efficiently with fetal hemoglobin than adult hemoglobin, due to a change in a single amino acid (residue 143) found in the 2,3-BPG 'binding pocket': from histidine to serine, leading to greater oxygen affinity.