Fab fragments are the antibody binding regions of an antibody. An IgG immunoglobulin contains two, comprising the arms of the classic Y shape. The Fab fragments can be isolated from the Fc portion, and from each other, by papain digestion. This results in a monovalent (one binding site) antibody.
Monovalent Fab fragments of affinity-purified secondary antibodies are offered to block endogenous immunoglobulins in tissue sections or on cell surfaces, to cover (block) immunoglobulins when double labeling primary antibodies from the same host species, or to Fab-label primary antibodies prior to incubation with the experimental sample. They can be used for these purposes because each Fab fragment has only a single antigen binding site (i.e. they are monovalent), and they are non-precipitating.
Divalent (whole IgG or F(ab') 2 fragment) antibodies are not recommended for these purposes because they have two antigen binding sites. After binding endogenous IgG or the first primary antibody, some antigen binding sites on a divalent secondary antibody may remain open, which could capture a primary antibody introduced in a subsequent step. Capture of the primary antibody would allow detection of that primary by a labeled secondary antibody, resulting in unwanted background staining or apparent signal overlap. Use of monovalent Fab fragments avoids these unwanted results.
Selected literature references:
We also have a limited inventory of DyLight™ 488 / 549 / 594 / 647, Cy™2, Cy™5, and Texas Red® conjugated secondary antibodies.