The monoclonal antibody M13/12 recognizes human complement factor B/Bb. Factor B is an acute phase protein. The level of factor B rises during inflammation. Complement factor B is a single-chain molecule of 764 amino acids (with a molecular weight of 90 kD), including a leader peptide of 25 amino acids. Complement factor B provides the catalytic subunit of the C3/C5 convertase that replaces the complement pathway. The assembly of C3 convertase (C3bBb) requires the binding of complement factor B and C3b (C3.H2O) and factor D-mediated cleavage of the bound complement factor B leading to the release of Ba (MW 33 kD). 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 the invertase. After cutting C3, C5 convertase ((C3b)2Bb) is formed. The Bb fragment may be regarded as a better indicator of the complement activation pathway than Ba, because impaired renal filtration does directly affect Ba levels. Compared with normal people, Bb in patients with chronic renal failure (CRF) increased by 2.2 times, while the plasma concentration of Ba in CRF was 8.4 times higher, and the plasma concentration of Ba in patients with end-stage renal disease (ESRD) was 16 times higher. In addition to complement activation, factor B fragments also participate in other immune functions.