In an experiment, bacteria are inoculated into aliquots of normal human blood that have been treated with an anticoagulant. It is observed that the bacteria are either phagocytized by neutrophils or undergo lysis. Which of the following blood plasma components is most likely to facilitate these effects?

Activation of complement may occur via microbial cell wall components such as polysaccharides (alternative pathway) or mannose (lectin pathway), or antibody attached to surface antigens (classic pathway). A variety of complement components are generated, including complement C5a, a neutrophil chemoattractant; complement C3b, an opsonin; and complement C5-9, the membrane attack complex. The remaining options are more closely associated with coagulation. Fibrin is generated by the coagulation system, but not with anticoagulation. Kallikrein may aid in generation of bradykinin and plasmin, but participates just in complement C5a generation. Plasmin is generated from plasminogen and helps lyse clots. Thrombin is generated by the coagulation cascade.