Used in immunological research to quantify complement fragment C3d levels in biological samples. Helps assess activation of the complement system, particularly in studies related to autoimmune diseases such as systemic lupus erythematosus (SLE). Enables researchers to investigate immune complex formation and complement-mediated tissue damage. Commonly applied in serum or plasma analysis to monitor disease progression or therapeutic response in experimental models and clinical research. Supports high-throughput screening in drug development targeting complement pathways.