Assays
There are no convenient assays for the biological activity of Ba. Several companies produce ELISA kits for measuring Ba levels in blood samples (Dodds, A.W. and Sim, R.B. (1997)).
Applications
Split products of factor B in plasma are indicative of activation of the alternative pathway in vivo. ELISA kits for measurement of Ba and Bb are commercially available.
These have been used in numerous human and animal studies (Lynch, A.M., et al. (2008)). See In vivo section below.
In vivo
The average concentration of factor B in blood is 200 µg/mL (range 170-258 µg/mL) in human plasma. Factor B is an acute phase protein whose plasma levels increases during inflammation. The fragments Ba and Bb are released upon activation of the alternative pathway and circulate in blood until cleared (Lynch, A.M., et al. (2008)).
Baseline levels in normals have been reported to be 29 ng Ba/mL (0.015% of the factor B concentration). The concentration of fragment Ba in patients with tubular proteinuria was found to be elevated more than 100-fold (4800 ng/mL)(Oppermann, M. et al. (1991)). There was a minimal increase in the plasma concentration of fragment Bb in these patients.
Genetics
The gene for factor B is located on human chromosome 6p21.3 within the MHC class III region between the class I and class II regions. The factor B gene lies between the larger gene for C2 (to which it is highly homologous) and genes for C4A and C4B. The gene is composed of 18 exons and spans 6 kb.
Deficiencies
No natural deficiencies of factor B have been identified in humans or animals.Mice deficient in factor B (B-/- mice), compared to wild-type, exhibit much lower or no pathology in a wide variety of diseases where alternative pathway activation is the cause of or exacerbates the pathology of these diseases. See Diseases section below. Acquired and secondary deficiencies do occur in humans. Human factor I deficiencies exhibit very low factor B levels due to the fact that C3b is not inactivated in the absence of factor I and C3b accumulates in blood. This results in binding of factor B, cleavage by factor D and rapid release of fragments Ba and Bb. Transfusions with normal plasma or reconstitution with factor I temporarily stop or slow production of these fragments.
Precautions/Toxicity/Hazards
This protein is purified from human serum and therefore precautions appropriate for handling any blood-derived product must be used even though the source was shown by certified tests to be negative for HBsAg, HTLV-I/II, STS, and for antibodies to HCV, HIV-1 and HIV-II.
Hazard Code: B WGK Germany 3
MSDS available upon request.