complement terminology question

[Aclamity]

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This is probably a really dumb question, but I've missed a few questions regarding complement because I didn't really understand what the terminology meant.

When people say "there is low C3 in the serum" is that implying the C3 is being used up elsewhere (because of complement activation)? For example, when they say "There is low levels of C3 in APSGN," what exactly does that mean?

How about when you have a swollen joint, like in RA, and they say "low C3 in the synovial fluid," what does that mean? Does that mean the C3 is low because the complement is being activated to C3a & C3b? Or does it mean there's simply no C3 in the joint, period?

ughhh

 

[CrimsonMirage]

Low C3 just means a low AMOUNT of C3 from the complement cascade is present. C3 is often low in autoimmune diseases like SLE because your body is attacking itself, thereby depleting the measured C3 levels.

Usually in nephritic conditions, the low C3 is because IgG and C3 are depositing in the glomeruli.

In rheumatoid arthritis, you have low C3 only in synovial fluid but normal C3 in the blood because the joints are the major site of autoimmunity in this disease.

Hope that helps! 🙂