Complement component 3 (C3)

What Is Complement Component 3 (C3)?

Complement Component 3 (C3) is a protein found in the blood that helps the immune system fight off infections, autoimmune diseases, and cancer. It is part of a larger system called the complement system, which consist of more than 20 proteins.

C3 plays an important role in identifying and destroying foreign organisms, such as bacteria and viruses. It also helps the body combat autoimmune diseases, such as lupus or rheumatoid arthritis, which are caused by the body mistakenly attacking its own healthy cells.

Preperation for Complement Component 3 (C3) Test

No preparation is required for a C3 blood test. Simply inform your doctor of any medications you are taking and any allergies you have. It is also important to tell your doctor if you are pregnant or planning to become pregnant.

Procedure for Complement Component 3 (C3) Test

During a C3 test, the doctor or lab technician will draw a blood sample, usually from a vein in your arm. The blood sample may be sent to a laboratory for testing.


  • C3a: This complement component is an anaphylatoxin which helps with inflammation.
  • C3b: C3b plays an important role in the capture and destruction of invading pathogens, by tagging them for destruction by other immune system components.
  • C3c: helps to initiate the classical pathway of complement activation which leads to lysis of cells.
  • C3d: helps to activate partial assembly of the membrane attack complex.


The C3 blood test is usually a safe procedure and carries no associated risks for healthy individuals. However, those taking medications may be at risk for complications.

Why Take the Complement Component 3 (C3) Test?

A C3 test is usually performed to identify and diagnose autoimmune disorders or infections. It can also be used to monitor the progress of treatment of certain autoimmune disorders.

When to Take the Complement Component 3 (C3) Test?

Your doctor may order a C3 test if you are experiencing signs and symptoms of a serious infection or autoimmune disorder. These may include fever, joint pain, fatigue, rash, and muscle aches. If a particular infection has already been identified, then the C3 test may be used to monitor its progress and effectiveness of treatment.