Streptavidin is a ～60 kDa homotetramer. Streptavidin binds four molecules of biotin with the highest affinity. The binding affinity of biotin to streptavidin is one of the highest reported for a non-covalent interaction to date, with a KD ～ 0.01 pM. Streptavidin has an immunosuppressive role[2].In Vitro: Streptavidin (2-200 μg/mL) significantly suppresses the proliferation of Lewis T cells to WF by 76%-83% compared with untreated responders.In Vivo: Streptavidin (8-80 mg/kg) treatment is effective in prolonging rat cardiac allografts in the high-responder WF-to-Lewis combination[2].