Secondary antibody is especially efficient in immunolabeling. In immunolabeling, 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.hen, 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.