What is C4A (Complement Component 4 A)?

What is the complement system?

The complement system refers to a series of >20 proteins, circulating in the blood and tissue fluids.

Most of the proteins are normally inactive, but in response to the recognition of molecular components of microorganisms they become sequentially activated in an enzyme cascade – the activation of one protein enzymatically cleaves and activates the next protein in the cascade.

  • The complement system is a group of proteins that move freely through your bloodstream.
  • It plays a critical role in inflammation and defense against certain bacterial infections.
  • Is also activated during reactions against incompatible blood transfusions, and during the damaging immune responses that accompany autoimmune disease.
  • Deficiencies of individual complement components or inhibitors of the system can lead to a variety of diseases.

What is the Complement C4 test?

The complement C4 test is one of the most frequently used complement component tests. Your doctor may order a complement C4 test if you’re experiencing symptoms that indicate an autoimmune disease. These symptoms may include:

  • extreme fatigue
  • muscle pain
  • joint pain
  • sudden, unexpected weight loss
  • muscle weakness
  • muscle paralysis

The complement C4 test  is also sometimes used to monitor protein levels in people who have already been diagnosed with an autoimmune disease.


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The information on is NOT intended to replace a one-on-one relationship with a qualified health care professional and is not intended as medical advice.

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