Hi! Do you mean C3a and C4a complement levels? You can do a search on here and find a lot on them. It was useful for me at one point because most of my labs were testing normal. And then suddenly I got a reading of I think 26,000 for my C4a (it shouldn't be over 2800). Dr. Shoemaker has done research on these levels, you can do an online search and see videos of him talking about it, how it corresponds with mycotoxin levels and correlates with HLA genotypes (that is, genetic testing). So for me, it makes sense that I have a hampered ability to create antibodies to mold (as discovered in my HLA testing). It told me that I needed to test my current environment for toxins.
C3a and C4a reportedly can identify if you are struggling with Lyme disease or other mycotoxins such as mold. There are many things that reportedly can help to lower these levels.
HLA testing can be helpful too.