Properdin is not factor B. Properdin is a distinct plasma glycoprotein that positively regulates the alternative complement pathway by binding and stabilizing C3/C5 convertases (C3bBb). Factor B is a separate protein that binds C3b and is cleaved by factor D into Bb (part of the convertase) and Ba. Properdin deficiency raises risk of Neisseria infections.

Complement factor B is a protein of the alternative complement pathway. It binds C3b and is cleaved by factor D into Ba and Bb; Bb joins C3b to form the C3 convertase (C3bBb), amplifying C3 cleavage and promoting opsonization, inflammation and membrane-attack complex formation. Ba has regulatory roles. Factor B is mainly produced in the liver and is essential for innate immune defense.

Properdin deficiency causes recurrent, often severe invasive Neisseria infections, especially meningococcal disease. Symptoms include sudden fever, headache, neck stiffness and photophobia (meningitis), rapid-onset septicemia with chills, hypotension, tachycardia, petechial or purpuric rash, and disseminated intravascular coagulation. Episodes are recurrent and can be fulminant; affected males in families may show early, repeated meningococcal infections.

Factor B is a complement-system protein in the alternative pathway. Cleavage by factor D produces Ba (a regulatory fragment) and Bb (sometimes written as “BB”), the enzymatically active fragment. Bb binds C3b to form the C3 convertase (C3bBb), amplifying C3 cleavage and driving opsonization and inflammation. Dysregulation can increase infection risk or cause complement‑mediated disease.

Factor B and factor D are proteins of the complement system’s alternative pathway. Factor B binds to surface-deposited C3b and, when cleaved by factor D, yields Ba and Bb; Bb is a serine protease that forms the C3 convertase (C3bBb), amplifying C3 activation. Factor D is a circulating serine protease that specifically cleaves C3b-bound factor B, initiating and regulating activation.

Antihemophilic factor B is another name for coagulation Factor IX — often called the Christmas factor or the plasma thromboplastin component (PTC). It’s a vitamin K–dependent clotting protein that participates in the intrinsic coagulation pathway; inherited deficiency of Factor IX causes hemophilia B, characterized by prolonged bleeding and easy bruising. Treatment may include replacement therapy with Factor IX concentrates.