Complement-mediated early clearance of Haemophilus influenzae type b from blood is independent of serum lytic activity.

The role of complement-mediated serum lytic activity in the clearance of encapsulated Haemophilus influenzae type b was studied by comparing clearance of bacteria from blood in normal mice with that in congeneic C5-deficient animals and mice depleted of complement by using cobra-venom factor. The clearance of organisms from blood by C5-deficient and C5-sufficient normal animals was similar. C3 depletion and the inability to fix C3 to the surface of bacteria were, however, associated with impaired clearance of organisms during the first 24 h after intravenous bacterial challenge. These studies suggest that complement-mediated opsonization rather than bacteriolysis is important in determining early clearance of H. influenzae type b from the bloodstream.