Biospecific recognition between a target macromolecule and an immobilized ligand is the defining retention principle of affinity chromatography. The ligand—an inhibitor, substrate analog, antibody, lectin, or metal chelate—is covalently attached to an agarose or other hydrophilic bead, often through a spacer, so that the complementary protein is bound from a mixture while unbound species are washed away; the complex is then dissociated by a competing ligand or a change of pH or ionic strength ^[1]. Cyanogen-bromide and related activations convert hydroxyl groups on the matrix into reactive intermediates that couple primary amines of the ligand ^[2]. Metal-chelate affinity chromatography is a related subclass in which immobilized metal ions, rather than an organic biospecific ligand, coordinate surface histidine and other residues ^[3]. Conceptual boundaries exclude ion-exchange and hydrophobic-interaction chromatography that lack a recognition ligand, and they exclude free-solution affinity precipitation. Essential features are a chemically defined immobilized ligand, a reversible binding equilibrium, and a matrix whose nonspecific adsorption is low enough that biospecificity dominates retention ^[4].