Fab and F(ab')2 fragments are two types of fragmented secondary antibodies that are commonly used in various applications. Here are some key points about F(ab) fragments:

• F(ab) fragments are monovalent antibody fragments that contain only one antigen-binding Fab portion and lack the Fc region. They are generated by papain digestion of whole IgG antibodies to remove the Fc region and the entire hinge region.
• F(ab) fragments have a molecular weight of about 50 kDa, which is smaller than whole IgG antibodies and F(ab')2 fragments. This allows for better penetration into tissues and reduces nonspecific binding to Fc receptors on live cells or to Protein A/G.
• F(ab) fragments are useful for blocking endogenous immunoglobulins on cells, tissues, or other surfaces, and for blocking the exposed immunoglobulins in multiple labeling experiments using primary antibodies from the same species. They can also be used to Fab-label primary antibodies prior to incubation with the experimental sample.
• F(ab) fragments are monovalent and have lower avidity than F(ab')2 fragments, which may affect their sensitivity and specificity in certain applications.