The alternative pathway (AP) is antibody independent and relies on native C3 undergoing minimal spontaneous hydrolysis. Hydrolyzed C3 binds factor B. Factor B, when bound to hydrolyzed C3, is cleaved by factor D into Ba and Bb. Hydrolyzed C3Bb is responsible for a constant low level of C3 cleavage into C3b. If C3b binds to an appropriate surface, factor B will bind with C3b to form C3bBb, a highly efficient C3-cleaving enzyme. This overall series of successive proteolytic steps is enhanced by the serum protein properdin, which stabilizes protein:protein interactions during the process. Factor H is essential in controlling the function of the alternative pathway by inhibiting the formation of and degrading C3bBb. Deficiencies of alternative pathway-specific components are rare, and usually lead to an increased frequency of Neisseria infections.
Primary immunodeficiency diseases: an update from the International Union of Immunological Societies Primary Immunodeficiency Diseases Classification Committee.
