Antibodies are small proteins that circulate in the bloodstream. They are part of the body's defence (immune) system and are sometimes called immunoglobulins. They are made by a type of white blood cell (a B lymphocyte). Antibodies attach to proteins and other chemicals in the body, which they recognise to be not normally found in the body ('foreign'). The foreign proteins and chemicals that antibodies attach to are called antigens.
This is a companion discussion topic for the original article at https://patient.info/allergies-blood-immune/antibody-and-antigen-tests