The role of complement in the induction and regulation of immune responses.

The vertebrate host, constantly threatened and invaded by various pathogens, has evolved two mechanisms of resistance. The first and phylogenetically oldest mechanism is the non-specific protection provided by complement alone or in concert with phagocytes such as neutrophils and macrophages. The second and evolutionarily later mechanism is the specific immune response involving antibodies, lymphocytes and macrophages. Both mechanisms serve the same function — to protect the host against pathogens.