Hb is a good buffer because of:: (A) Histidine residues, (B) Protein nature, (C) Acidic nature, (D) Iron molecule
Answer is A. Harper mentions …‘Hemoglobin also functions in CO2 and proton transport from tissues to lungs. Release of O2 from oxy-Hb at the tissues is accompanied by uptake of protons, due to lowering of the pKa of histidine residues.’

Decarboxylation of histidine to histamine is catalyzed by a broad-specificity of aromatic L-amino acid decarboxylase.
Histidine compounds present in the human body include ergothioneine, carnosine, and dietary anserine.