A measurement of the amount of an immunoglobulin molecule possessing a specific amino acid sequence that binds to a specific antigen, a substance introduced into an organism which initiates an immune response including the production of the very antibodies which bind to it in an effort to destroy it, in a specified sample of blood, the fluid that circulates through the heart, arteries, capillaries and veins carrying nutrients and oxygen to the body tissues and metabolites away from them.