Used in immunological research to detect and quantify C4d, a stable breakdown product of complement component C4, in biological samples. The kit helps assess activation of the classical and lectin pathways of the complement system, which plays a key role in immune response and inflammation. Commonly applied in studies of autoimmune diseases such as systemic lupus erythematosus (SLE), transplant rejection—especially in kidney and heart transplants—and in diagnosing conditions involving complement-mediated tissue damage. The assay provides a sensitive and specific method to measure C4d levels in serum, plasma, or tissue eluates, supporting both clinical research and laboratory investigation of complement activation.