The MicroVue Bb Enzyme Immunoassay measures the amount of Bb present in human plasma and serum. Because Factor B is unique to the alternative complement pathway, Bb is an excellent marker for allternative pathway activation.

The alternative complement pathway provides innate protection against microbial agents in the absence of specific antibody. The activation of this complement pathway can be triggered by a variety of different chemical moieties and substances including LPS, bacterial lipids and polysaccharides, virally infected mammalian cells as well as transformed cancer cells.

Centrally important to the alternative pathway is the zymogen Factor B. Factor B is a 93 kD protein which is converted to an active proteolytic enzyme via a two step reaction involving C3b and Factor D. Factor B binds to C3b (or hydrolyzed C3) either in fluid phase or on a membrane or other surface. In the second step, Factor D cleaves the bound Factor B to yield Ba (33 kD) and Bb (60KD) The resulting bimolecular complex is the alternative pathway convertase and is capable of cleaving additional C3 or, together with a second C3b molecule, C5.

The Bb fragment may become dissociated from the C3 or C5 convertases either through spontaneous decay or via the action of Factor H or CR

Since the Bb molecule is unique to the alternative pathway of complement, evels of Bb provide an excellent marker for complement activation via this pathway.

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