Hb is a good buffer because of:
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.
Histidine residues
Protein nature
Acidic nature
Iron molecule