An antibody is a protein produced by the body’s immune system when it detects harmful substances, called antigens. Antibodies may be produced when the immune system mistakenly considers healthy tissue a harmful substance.
Secondary antibody is especially efficient in immunolabelling. In immunolabelling, primary antibody’s Fab domain binds to an antigen and exposes its Fc domain to secondary antibody. Then, secondary antibody’s Fab domain binds to the primary antibody’s Fc domain. Since antibody’s Fc domain is constant within the same animal class, only one type of secondary antibody is required to bind to many types of primary antibodies.