Introduction
Complement component C1r is a zymogen of a serine protease that combines with C1q and C1s to form C1, the first component of the classical complement pathway. C1r is a dimer of identical chains and a key mediator of innate immunity. Each precursor contains a 17-amino acid leader peptide, followed by a mature 688-amino acid protein (1). Upon C1q binding to the surface of pathogens, the activated C1r is cleavage into two chains, A and B, connected by disulfide bonds. The non-catalytic amino-terminal C1r A chain (heavy) has 446 amino acids residues (Mr 51 kDa) and contains a growth factor domain and two internal repeats. The catalytic C1r B chain (light) contains 242 amino acids (Mr 27 kDa) and is homologous to the trypsin family of serine proteases. The activated C1r is able to activate C1s which in turn activates C2 and C4, leading to the formation of the membrane attack complex and the elimination of the target (2-3). C1r deficiency is associated with autoimmune diseases such as systemic lupus erythematosus (4).

Principle of the Assay
The AssayMax Human Complement C1r ELISA kit is designed for detection of C1R in human plasma, serum, saliva, urine, milk, and cell culture samples. This assay employs a quantitative sandwich enzyme immunoassay technique that measures C1r in less than 4 hours. A polyclonal antibody specific for C1r has been pre-coated onto a microplate. C1r in standards and samples is sandwiched by the immobilized antibody and a biotinylated polyclonal antibody specific for C1r, which is recognized by a streptavidin-peroxidase conjugate. All unbound material is then washed away and a peroxidase enzyme substrate is added. The color development is stopped and the intensity of the color is measured.