Complement System

The complement system is part of your immune system that defends your body against injury and foreign invaders like bacteria and viruses that can make you sick. Your complement system activates proteins that work with your immune system to keep you healthy.


What is the complement system?

The complement system is part of your body’s immune system that cleans up damaged cells, helps your body heal after an injury or an infection and destroys microscopic organisms like bacteria that make you sick. Your complement system is the front line of defense for your immune system.

Your body’s complement system gets its name because it “complements” or enhances your body’s infection-fighting cells and cells.


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What does the complement system do?

The complement system is a connection to your body’s defense system to protect you from anything that can damage your body. One of the most common things your complement system does when it activates your defense system is to protect your body from foreign invaders like bacteria that cause an infection or an illness.

When your defense systems activate, your complement system:

  • Targets foreign invaders.
  • Activates inflammation to prevent infection.
  • Removes foreign invaders from your body.

What makes up your complement system?

Plasma proteins, which are in your blood and can be seen with a microscope, make up your complement system. You have about 50 plasma proteins in your body’s complement system.

How do complement system proteins activate?

The proteins that make up your complement system are inactive or turned off until a trigger turns them on. Triggers could include an injury to your body or bacteria that enter your body. Proteins turn on in a line that’s similar to dominos. One protein will turn on, which activates the next protein in line to turn on, just like how one domino falls onto another domino to create a chain reaction. This chain reaction is called a cascade. When each protein activates, it can perform its function to protect your body and turn on the next.



Where is the complement system located?

The proteins that make up your body’s complement system are created in your liver and move to your tissues and blood.

How are proteins important to the complement system’s structure?

About 50 proteins make up your body’s complement system. These proteins circulate throughout your blood and tissues. When the proteins activate or turn on, they connect with foreign invaders and work to destroy them to prevent an illness or injury from harming you.


Conditions and Disorders

What are common conditions that affect the complement system?

The complement system is an important part of your immune system and functions to prevent infection and illness. If your complement proteins aren’t working as expected, or if you don’t have enough of them (complement deficiency), you’re considered immunocompromised and your body is at risk of infections that can be very serious (sepsis).

If your complement proteins are working too hard and activating too frequently, you may be at risk for several autoimmune or inflammatory conditions. Common conditions that your complement system can be involved in include:

What are common signs or symptoms of complement system conditions?

If your complement system isn’t functioning properly, your immune system will be weaker than it should be. Symptoms of complement system conditions include:

What are common tests to check the health of my complement system?

A complement test is a blood test where a sample of blood is drawn from your vein that measures the amount and function of proteins in your complement system. Complement tests can measure proteins individually or in total to look for any weakness or overactivity. Complement tests help identify:

  • Why you get frequent infections.
  • Causes of autoimmune disease and effectiveness of treatment.
  • The overall health of your immune system.

Other important tests if your immune system is not working properly include a complete blood count and immunoglobulin (antibody) levels.

What are common treatments for complement system conditions?

Treatments for complement system conditions focus on managing symptoms and treating the underlying complement deficiency or over-activity. Treatment options include:


Your complement system is part of your immune system. There are several ways you can keep your complement system healthy.
Your complement system is part of your immune system. There are several ways you can keep your complement system healthy.

How can I keep my complement system healthy?

You can’t directly change how your complement system works, but you can keep your whole immune system healthier by:

  • Avoiding injury.
  • Eating a well-balanced diet.
  • Exercising regularly.
  • Practicing good hygiene.
  • Getting enough sleep.

A note from Cleveland Clinic

In the same way that your best friend supports you, your complement system enhances your cells so they can be more successful in defending your body from foreign invaders and injury. Keep your complement system healthy by caring for your whole body, like sleeping enough, using proper hygiene and eating a well-balanced diet.

Medically Reviewed

Last reviewed on 06/28/2022.

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