An antibody was engineered to competitively inhibit binding between protective antigen (PA) of anthrax and the anthrax receptor on the membrane of the host (human) cells. The biological studies demonstrated that the PA-Receptor dissociation constant is Kd=170 x 10-12M. What would the inhibition constant have to be in order to reduce r, the moles of PA ligand bound per mole receptor, by 98% at a blood PA concentration of 10-8 M and an antibody concentration of 10-6 M.