Library · Physiology
Complement system
Make it clearer
The complement system is a group of about 40 proteins that circulate in the serum, the liquid part of the blood. They work together with innate immunity, the fast defense we are born with, and adaptive immunity, the kind that learns from each germ, to clear pathogens.
They do not act on their own: they act in a cascade, a domino effect in which each piece pushes the next one. There are three ways to tip the first domino, three pathways: the classical, the lectin and the alternative. All three arrive at the same point, an enzyme called a convertase that splits the protein C3 into two pieces, C3a and C3b.
From there come the effects of complement. C3b acts as an opsonin, which means it sticks to the microbe and tags it, and that prompts the scavenger cells to eat it: that is phagocytosis. C3a and C5a are anaphylatoxins, and their job is to activate neutrophils, monocytes and mast cells, which are defense cells. And when C5b is deposited, the assembly of the membrane attack complex begins.
In brief
Complement is a set of about 40 serum proteins that work with innate and adaptive immunity to clear pathogens.
They act in a cascade through three pathways, the classical, the lectin and the alternative, which converge on a convertase that cleaves C3 into C3a and C3b.
It is a domino effect: each piece pushes the next one.
C3b acts as an opsonin and promotes phagocytosis; C3a and C5a are anaphylatoxins and activate neutrophils, monocytes and mast cells; C5b deposition starts the membrane attack complex.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingPhysiology, Complement Cascade, StatPearlsPublished
This page explains the topic in general terms; it does not tell anyone what treatment to follow.
Updated