After an antibody has been fragmented by papain, which term denotes the antigen-binding fragment?

The term that denotes the antigen-binding fragment after an antibody has been fragmented by papain is "Fab." When antibodies undergo proteolytic cleavage by enzymes such as papain, they are cleaved into three main fragments: two Fab fragments and one Fc fragment.

Each Fab fragment consists of the variable region and part of the constant region of the heavy and light chains, and it is responsible for specific binding to the antigen. This is crucial in immunology, as it is the Fab portion of the antibody that directly interacts with antigens, enabling the immune response.

The Fc fragment, on the other hand, is the portion of the antibody that does not bind antigen but is involved in effector functions, such as binding to Fc receptors on immune cells. The Fab'2 fragment refers to a dimer of Fab fragments, which can occur under different conditions and does not represent the simple antigen-binding capability of the individual Fab fragments. Thus, the designation "Fab" is specifically used to describe the antigen-binding domains generated by papain cleavage.