Complement factor B, a 93kDa glycoprotein, is a component of the alternative complement pathway. Complement factor B circulates in the blood as a single chain polypeptide. Upon activation of the alternative pathway, it is cleaved by complement factor D yielding the noncatalytic chain Ba and the catalytic subunit Bb. The active subunit Bb is a serine protease which associates with C3b to form the alternative pathway C3 convertase. The alternative pathway is associated with inflammation, immunologic regulation, and bacterial cytotoxicity. In addition to its roles in the activation of the alternative pathway and enhancing bacterial phagocytosis by macrophages, Complement factor B may also play a role in B cell proliferation, monocyte cytotoxicity, macrophage spreading, immunosuppression, and apoptosis. Complement factor B has been shown to be involved in many diseases such as septic shock, stroke, systemic lupus erythematosus, Alzheimer’s disease, and multiple sclerosis.