Complement System

The complement system consists of about 20 plasma proteins, working together to fortify the body's defense against infections through opsonization, inflammation, and cell lysis.

These proteins, labeled C1 through C9, along with other regulatory proteins, typically circulate in an inactive state in the blood.

The complement system cascade can be activated by the classical, lectin, and alternative pathways.

In the classical pathway, the antigen-antibody complex binds and activates complement protein C1 to rapidly stimulate downstream complement cascade.

In contrast, the lectin pathway is triggered by the attachment of carbohydrate-binding lectin proteins to pathogens.

The alternative pathway is stimulated by the exposure of complement proteins P, D, and B to specific molecules found on pathogen surfaces.

All activation pathways eventually lead to the cleavage of C3, a central component, into C3a and C3b.

C3a plays a role in inflammation, while C3b is involved in opsonization and activation of the other complement proteins.

These activated proteins assemble on the pathogen surface, forming a membrane attack complex and creating pores that lead to cell lysis.