Used in immunological research to detect and quantify C4b protein levels in biological samples. Helps assess activation of the complement system, particularly in studies related to autoimmune diseases such as systemic lupus erythematosus (SLE), immune complex disorders, and transplant rejection. Supports investigation of classical and lectin pathways of complement activation. Commonly used in clinical labs and research settings to evaluate immune response mechanisms and monitor disease progression or therapeutic interventions involving complement regulation.