Complement refers to a series of plasma proteins that become activated in a cascade fashion, similar to how the coagulation pathway becomes activated. They are called "Complement" because they complement or enhance the immune response. Three different triggers can activate this pathway: the pathogen surface itself (the "alternative pathway"), mannose-binding lectin (the "lectin pathway"), or antibody and C-reactive protein attached to the microbe surface (the "classical pathway").
The proteins' activation involves a cleavage that produces the "a" and the "b" components. The "a" components are called anaphylatoxins and recruit more inflammatory cells, thereby increasing the innate response. The "b" components go on to activate the next protein.
The effects of Complement are far-reaching in the immune response:
- The "3Cb" component coats the pathogen surface, thereby facilitating uptake and phagocytosis by phagocytic cells. This process is called opsonization. Many cells have receptors for complement components.
- As mentioned above, the "a" components recruit inflammatory cells.
- The final complement components act together to directly perforate the microbe membrane itself, leading to pathogen death. This group of complement proteins is known as the "membrane attack complex."
