The antigen binding domain refers to a specific region or portion of an antibody or a T-cell receptor that is responsible for recognizing and binding to a specific antigen. It is a structurally distinct part of the antibody or receptor that allows for the specific recognition of the antigen's unique features or epitopes. The antigen binding domain plays a crucial role in immune responses by facilitating the interaction between the immune cells and the antigens, leading to various immune reactions and immune-mediated processes.