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Name: Gaye G.
Status: Student
Age: 20s
Location: N/A
Country: N/A
Date: N/A 

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 osmotic pressure.

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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