Affinity chromatography uses a specific biological ligand (e.g. Protein A for IgG antibodies, metal chelate for His-tagged proteins, substrate analogue for enzymes) immobilised on a matrix. The target binds specifically and tightly; impurities flow through. Elution is achieved by pH change, salt, or competing ligand. This one-step technique can achieve >1000-fold purification — essential for monoclonal antibody manufacturing.