C3 (Complement Component 3)

Complement Component 3 (C3) is a protein that forms part of the complement system, a group of proteins in the blood that work together as part of your immune defences. The complement system helps the body fight infection, clear away damaged cells, and support the wider immune response. C3 is one of the most important and abundant proteins in this system, and measuring its level can give useful insight into immune activity.

What it can help assess

• Autoimmune conditions: C3 levels can fall when the complement system is being actively used up, as happens in conditions such as lupus (SLE). It is often used to help monitor disease activity.
• Kidney conditions: certain types of kidney inflammation (glomerulonephritis) are associated with low C3 levels.
• Recurrent or unusual infections: low levels may point to a deficiency in the complement system that can leave the body more vulnerable to infection.
• Inflammation: as C3 is also an acute phase protein, levels can rise in response to infection or inflammation.

Why it matters

Measuring C3 helps give a picture of how active your complement system is. It is most often used alongside other tests, including Complement Component 4 (C4), to help investigate and monitor autoimmune and kidney conditions, and to support the diagnosis of complement-related immune disorders.