Name: Gaye G.
I would like to know how does
'complement system' (or complement molecules) work in the immune system?
how are they being activated and what is their importance for the
body? thank you.
The complement is a collection of proteins in the
blood that fight bacterial infections. They work in a
cascade manner, so that every next step is a
proliferation of the former. This accumulates into the
formation of a complex on the surface of invading
bacteria. The complex produces a pore (a hole) in the
bacteria so that they become leaky and die.
The complement is activated by the presence of
bacteria and also indirectly by other immune cells and
by antibodies. Complement is part of the innate immune
system which means it is non-selective and will attack
any invading bacteria. Antibodies are part of acquired
immunity and will only bind to specific bacteria.
If you want to read more about immunity and the role
of complement, go to
Dr. Trudy Wassenaar
curator of the Virtual Museum of Bacteria
The complement system gets its name because it "complements" the function
of antibodies in your immune system. Basically, it consists of a family of
proteins that function to kill an invader, such as a bacterial cell, that has
been bound by an antibody.
Roughly 20 different proteins make up the complement system. They are
made in the liver and released into your circulatory system, where they
normally float around in an inactive state. If a bacterial cell invades your
body, some of the B-cells of your immune system will be stimulated to make
and release proteins called antibodies which bind to some surface portion(s)
of the invading cell.
When an antibody binds to a microbial cell, the antibody (presumably)
undergoes a conformational (shape) change that allows the first proteins in
the complement system to bind. This starts a "cascade" where additional
complement proteins are enabled to bind, and so on, until a large complement
complex is bound to the surface of the invading cell.
The final members of the complement complex form a "membrane attack
complex" that can actually create small pores or holes in the bacterial cell
membrane. These holes in the membrane make the cell "leaky" and cause the
death of the invading cell, as it swells and bursts due to changes in its
There are two other ways that the complement system helps the immune
system. First, some complement proteins can bind directly to certain
polysaccharides (large protein/sugar complexes) found on the surface of many
bacterial cells. After they bind, the rest of the scenario plays out as
described above, without the need for an antibody to bind.
Lastly, complement proteins can bind to receptors on the macrophages of
your immune system, enhancing their ability to phagocytose (basically,
encircle and digest) the invading cell.
Paul Mahoney, Ph.D.
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Update: June 2012