Human Complement C4d ELISA Kit (A77779) employs the sandwich enzyme immunoassay technique for the quantitative measurement of human Complement C4d in serum, plasma, tissue homogenates, and other biological fluids. An antibody specific for Complement C4d has been pre-coated onto a 96-well microtiter plate. The standards and test samples are added into the wells and the Complement C4d present in each sample is bound to the wells by the immobilized antibody. Following incubation, the wells are washed and then incubated with Biotinylated Anti-Complement C4d Antibody, which binds the captured Complement C4d present in each well. Following incubation, unbound biotinylated detection antibody is removed by washing, and an HRP-Streptavidin conjugate is added to the wells and the microtiter plate is incubated. Following incubation and washing, TMB substrate solution is then used to visualize the HRP enzymatic reaction by catalysis to produce a blue-coloured product that changes to yellow after addition of acidic stop solution. The density of yellow is proportional to the amount of Complement C4d captured in each well. The concentration of Complement C4d can then be calculated by reading the O.D. absorbance at 450nm in a microplate reader and referring to the standard curve.