PROPERDIN FACTOR B

Properdin and Factor B are central components of the alternative complement pathway. Factor B binds C3b to form the alternative-pathway C3 convertase (C3bBb), and properdin stabilises this complex. Both are essential for complement amplification, defence against encapsulated bacteria (especially Neisseria meningitidis), and have a role in immune-complex clearance.

Quantitative serum measurement is done by nephelometry. Properdin deficiency is an X-linked primary immunodeficiency causing severe meningococcal infection. Factor B deficiency is extremely rare. Both can be acutely consumed in alternative-pathway-mediated diseases.

These tests are ordered in two main contexts. First, primary immunodeficiency — properdin deficiency in a young male with recurrent severe meningococcal disease, particularly in family clusters. Second, alternative-pathway complement diseases — C3 glomerulopathy, atypical haemolytic uraemic syndrome (aHUS), dense-deposit disease, and membranoproliferative glomerulonephritis. In these conditions Factor B and properdin can be consumed, and detection of autoantibodies against the C3 convertase (C3 nephritic factor) supports the diagnosis.

Indian nephrologists increasingly include alternative-pathway complement testing in unexplained glomerular disease — particularly children and young adults presenting with persistent low C3 and normal C4. These conditions need specialist immunology and nephrology care; treatment may include anti-C5 therapy (eculizumab, ravulizumab) for aHUS.

No fasting required. Disclose any acute infection (transient consumption), pregnancy, anti-complement therapy (eculizumab), and any kidney disease. Sample stability is important — complement proteins degrade quickly at room temperature, so the sample should be processed and frozen within hours of collection if not measured immediately.