OKT3, also known as muromonab-CD3, is a monoclonal antibody that targets the CD3ε component of the T-cell receptor (TCR) complex. CD3 is essential for T-cell activation, signal transduction, and immune effector functions. By binding CD3, OKT3 triggers receptor internalization and functional suppression of T cells. OKT3 became the first FDA-approved monoclonal antibody, originally used to treat acute rejection in solid organ transplantation. Its mechanism involves transient activation followed by depletion or paralysis of T cells, effectively dampening harmful allo-reactive responses. However, its clinical use was limited by cytokine release syndrome and anti-mouse antibody formation. Beyond clinical application, anti-CD3 antibodies, including OKT3, are widely used in research to stimulate T-cell activation in vitro and to modulate immune responses in vivo. These reagents help dissect signaling pathways, differentiation programs, and immunoregulatory circuits. Thus, OKT3 remains a landmark molecule in immunology—both as a therapeutic pioneer and as a powerful experimental tool to explore T-cell biology.