Secondary antibodies bind to primary antibodies, which are directly bound to the target antigen(s). In immunolabeling, the primary antibody\'s Fab domain binds to an antigen and exposes its Fc domain to secondary antibody. Then, the secondary antibody\'s Fab domain binds to the primary antibody\'s Fc domain. Since the 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.