Complement is a major defense system of innate immunity and aimed to destroy microbes. There are three pathways of complement activation. The classical and lectin pathways are initiated by the binding of recognition proteins to specific targets. The classical pathway is activated by IgM, isotypes of IgG, and several other proteins such as C-reactive protein and serum amyloid P protein. The lectin pathway is initiated by the binding of mannose binding lectin to carbohydrate moieties found primarily on the surface of pathogens. In contrast to the other two pathways, the alternative pathway is continuously activated as a result of spontaneous hydrolysis of complement component C3. Because of this spontaneous activating capability, the alternative pathway requires continuous regulation. Endogenous cells and tissues protect themselves from uncontrolled activation by expressing complement regulatory proteins (especially Factor H) that are able control alternative pathway activation. An “activating” surface is, in large part, one without adequate regulatory protein function to control alternative pathway activation (such as the surface of pathogens). The auto-activation process starts with the spontaneous hydrolysis of C3, which results in the formation of C3(H2O). In turn, C3(H2O) binds complement factor B. Once factor B associates with C3(H2O), factor B itself changes conformation and can then be cleaved by the constitutively active serum protease factor D, generating Ba and Bb. The Bb fragment remains associated with the complex and can then, through its own serine protease domain, cleave additional C3 molecules, generating a form designated C3b. Once C3b is generated, it associates with factor B to generate more C3-convertase, resulting in the activation of the complement cascade. Factor B circulates in the blood as a single chain polypeptide. The human Factor B ELISA is to be used for the quantitative determination of Factor B in plasma samples.