Complement Component 3 (C3), the most abundant serum complement component, is a disulfide-linked 185kDa 1,637 amino acid glycoprotein which supports the classical, alternative, and lectin pathways of complement activation. C3 is proteolytically activated by C3-convertase to the anaphylatoxin C3a and the opsonizing agent C3b. Serum concentrations of C3 are increased during acute and chronic inflammation such as rheumatoid arthritis, and are decreased due to increased consumption or autoimmune disorders such as systemic lupus erythematosus. The sensitive quantitative measurement of total human C3 antigen in plasma, serum, urine, milk, saliva and cell culture samples is easily performed with this 96 well strip format ELISA kit. C3 in normal human plasma ranges from 0.9-1.9 mg/ml with an average concentration of 1.39 mg/ml. The assay measures total human C3 in the 0.02-10 ng/ml range. Samples giving human C3 levels above 10 ng/ml should be diluted in blocking buffer before use. For best results, dilute plasma and serum samples 1:100,000 to 1:1,000,000, saliva samples to 1:100, urine samples 1:2 to 1:10, and milk samples 1:1,000 to 1:10,000. Human C3 will bind to the capture antibody coated on the microtiter plate. After appropriate washing steps, biotin labeled anti-human C3 primary antibody binds to the captured protein. Excess primary antibody is washed away and bound antibody is reacted with streptavidin conjugated to HRP. Following an additional washing step, TMB substrate is used for color development at 450nm. Color development is proportional to the concentration of C3 in the samples. A standard calibration curve is prepared using dilutions of purified C3and is measured along with the test samples. All reagents and standards are provided in these ELISA kits.