Complement component 3, often simply called C3, is a protein of the immune system. It plays a central role in the complement system and contributes to innate immunity. In humans it is encoded on chromosome 19 by a gene called C3.

What does a C3 complement test for?

Complement C3 is a blood test that measures the activity of a certain protein. This protein is part of the complement system. The complement system is a group of nearly 60 proteins that are in blood plasma or on the surface of some cells.

What does low C3 complement mean?

If only your C3 complement level is low and all other complement components are normal, it's usually because of an inherited component deficiency. This makes it more likely that you will develop certain autoimmune disorders. More often, you will have low levels of several complement components at once.