In this example the need for blocking with a monovalent Fab fragment is obviated by incubating with the unlabeled primary antibody first. After the unlabeled primary antibody is detected with a labeled secondary antibody, open binding sites on the labeled secondary antibody must be blocked before incubating with the labeled primary antibody. This is done as shown below, by incubating with a source of IgG, such as normal serum, from the same species as the primary antibody.