C4 Antibody (Polyclonal)

C4 is a plasma protein that is key to the classical and lectin pathways of complement activation. This polypeptide is approximately a 200 kD andis primarily synthesized in single chains by liver hepatocytes. However, synthesis by epithelial cells of the genitourinary and intestinal tract and by phagocytic mononuclear cells has also been reported, and C4 generally circulates as a three-chain molecule instead of a single chain. The C4 protein contains 3 disulfide bonded subunits: α – 93 kD, β – 75 kD, and γ – 32 kD. The normal concentration of C4 in serum/plasma is approximately 400 μg/mL. Upon activation of the classical or lectin complement pathway, C4 is cleaved into C4a and C4b fragments. C4a is one of three distinct anaphylatoxins produced directly as a result of complement activation. Upon complement activation, the C4b fragment α-chain’s thioester bond becomes accessible to nucleophilic attack by target cell acceptor molecules or non-complement proteolytic enzymes. Such an attack results in a covalent ester bond between the C4b fragment and the target cell surface. This attachment provides the binding site for C2a. The binding of C4b and C2a forms the C3 convertase, C4b, C2a. After the C3 convertase has cleaved C3, the C3b fragment of C3 can also then bind to the C3 convertase, which forms the C5 convertase: C3b, C4b, C2a. Cell-bound C4 can interact with complement receptors (CR1) present on phagocytic leukocytes to mediate opsonization of the cell. This mediation response is inhibited by cleavage of the C4b fragment into C4c and C4d by C4b binding proteins and Factor I. The C4c oligopeptide is released from the target surface where the C4d remains bound.

The anti-human C4 polyclonal antisera was tested against normal human plasma by double immunodiffusion, one-dimensional immunoelectrophoresis, quantitative radial immunodifussion, and quantitative rocket immunoelectrophoresis. The antiserum was determined to be monospecific for C4 at varying concentrations.