The monoclonal antibody P21/15 recognizes human complement factor B/Ba. Factor B is an acute-phase protein. Levels of factor B increase during inflammation. Complement factor B is a single- chain molecule of 764 amino acids (MW 90 kD), including a leader peptide of 25 amino acids. Factor B provides the catalytic subunit of the C3/C5 convertases of the alternative complement pathway. Assembly of the C3 convertase (C3bBb) requires binding of factor B to C3b (C3.H2O) and factor D-mediated cleavage of bound factor B resulting in the release of Ba (MW 33 kD). The C3 convertase is stabilized by the binding of properdin. This provides a positive amplification loop for the classical and alternative complement pathways. Bb (MW 60 kD) is the serine protease element of this convertase. After cleavage of C3, the C5 convertase ((C3b)2Bb) is formed. The Bb fragment may be regarded as a better indicator of the alternative pathway of complement activation than Ba as impaired renal filtration does directly influence Ba levels. Whereas Bb is elevated 2.2-fold in chronic renal failure (CRF) patients, plasma concentrations of Ba are 8.4-fold higher in CRF and 16-fold higher in end-stage renal disease (ESRD) patients compared to normals.In addition to complement activation, factor B fragments participate in other immunological functions.