A pharmaceutical composition includes an isolated anti-Bb antibody or antigen binding portion thereof produced by a hybridoma cell line deposited under ATCC Accession Number PTA-8543 or an isolated anti-Bb antibody or antigen binding portion thereof that competitively inhibits binding of the antibody or antigen binding portion produced by the hybridoma cell line deposited under ATCC Accession Number PTA-8543 to the Bb segment of factor B and a pharmaceutically acceptable carrier.