"Severely reduced complement activation or "Severe complement deficiency" is what I would use for the translation of "sterkt nedsatt komplementsfunksjon"

The complement system consists of almost 60 proteins, approximately 30 of which are circulating blood proteins that work together to promote immune and inflammatory responses. Complement tests measure the amount or activity of complement proteins in the blood.

The complement system’s principal role is to help identify, destroy and remove foreign pathogens like bacteria and viruses, as well as damaged “self” materials (e.g., cells and proteins). The complement system is so named because it “complements” or aids the natural body defenses, such as antibodies. So, the complement system is also activated when the body makes antibodies, whether against itself or foreign invaders. The body makes antibodies against its own tissues that it thinks are foreign (autoantibodies) in various autoimmune diseases.

https://www.testing.com/tests/complement/

https://metodebok.no/index.php?action=topic&item=bVMtjQB4