Complement C4 details

Complement C4 is one of the core proteins in your complement system, part of your immune system's infection-fighting and immune-complex-clearing machinery. Low C4 is a common finding during active lupus flares, since complement proteins get consumed as they're activated by immune complexes, and is also seen in certain other immune-complex-related conditions and in some inherited complement deficiencies. C4 is typically tested alongside C3, since the pattern of both together — and how they trend over time — can help track disease activity in someone with a known autoimmune condition, or help work up a suspected new one.